Learning how to identify rocks and minerals can be a rewarding hobby and a useful skill for hiking, camping, and survival. Rock hunting can deepen your understanding of geology and the earth’s history, and of course, it is fun. Those science-minded nerds who love to collect rocks are called “rockhounds“.
A rockhound is a person who is interested in collecting rocks and minerals as a hobby. They might go out into the field to collect specimens or visit rock and mineral shows, searching for rare rocks, minerals, and gemstones to add to their collection. Some rockhounds might also be interested in lapidary, the art of cutting, shaping, and polishing stones. Some simply appreciate the natural beauty of rocks and minerals.
Use this Rockhound rock identification guide to help you identify rocks and minerals. Learn the types of rocks and general characteristics rock hounds use to identify a rock. Then run through our free rock identification guide to pinpoint your find.
Let Geek Slop know if you have any suggested additions to the guide, and of course, forward us your photos to add to the photo gallery.
Step 1 – Identify the type of rock
To identify a rock, a rockhound must first determine the type of rock – is it igneous, sedimentary, or metamorphic?
Igneous rocks
Igneous rocks like granite or lava are hard, uniform rocks with a smooth surface, possibly even a glassy texture. They usually have black, white, and/or gray minerals.
Igneous rocks are formed from the cooling and solidification of magma or lava. Magma is molten rock that is found beneath the Earth’s surface, while lava is molten rock that has erupted onto the Earth’s surface. When magma or lava cools and solidifies, it forms igneous rocks. The type of igneous rock that forms depends on the rate of cooling and the chemical composition of the magma or lava.
Igneous rocks are further classified into intrusive and extrusive rocks depending on whether they cool and solidify beneath the earth’s surface or on top of it. Intrusive rocks, also known as plutonic rocks, are formed through the slow cooling and solidification of magma deep within the Earth’s crust. They are typically coarse-grained and have a crystalline structure. Examples of intrusive rocks include granite, diorite, and gabbro. Rockhounds usually find igneous rocks in mountainous areas or within the cores of continents.
Extrusive rocks, also known as volcanic rocks, are formed through the rapid cooling and solidification of lava on the Earth’s surface. They often have a fine-grained texture and may contain air pockets or vesicles. Examples of extrusive rocks include basalt, pumice, and obsidian. These rocks are usually found near volcanic areas or within regions of tectonic activity.
Examples of igneous rocks
Examples of igneous rocks include:
- Basalt
- Obsidian
- Pumice
- Andesite
- Dacite
- Rhyolite
- Granite
- Gabbro
- Peridotite
- Scoria
- Tuff
- Phonolite
- Trachyte
- Komatiite
- Kimberlite
- Nephelinite
- Carbonatite
- Aplite
Sedimentary rocks
Sedimentary rocks like limestone or shale are rocks made from sediment that has hardened. They have sandy or clay-like layers and are usually brown to gray in color. Sometimes, they have fossils and marks from wind or water.
Sedimentary rocks are formed from the accumulation of sediment over time. Sediment consists of pieces of rock, mineral fragments, shells, and other debris that are transported by water, wind, or ice. As the sediment is deposited and compacted, it forms layers that eventually harden into rock.
Rockhounds usually find sedimentary rocks near bodies of water such as rivers, lakes, and oceans. They can also be found in areas where there has been a lot of volcanic activity or windy areas where sediment has accumulated over time.
Examples of sedimentary rocks
Examples of sedimentary rocks include:
- Sandstone
- Shale
- Limestone
- Conglomerate
- Siltstone
- Mudstone
- Arkose
- Chert
- Coquina
- Dolomite
- Breccia
- Evaporite
- Greywacke
- Micrite
- Oolite
- Travertine
- Tufa
- Ironstone
- Coal
Metamorphic rocks
Metamorphic rocks like marble are hard, with layers of light and dark minerals and a foliated texture. They come in various colors and often contain glittery mica.
Metamorphic rocks are formed from the transformation of existing rocks due to heat, pressure, or chemical reactions. These rocks are formed when existing rocks are subjected to high temperatures and pressure, which causes them to change their physical and chemical properties. The transformation can also occur due to chemical reactions with fluids that flow through the rock.
Metamorphic rocks can be classified into foliated and non-foliated rocks, depending on whether they have a layered or banded appearance. Foliation is a common feature of metamorphic rocks that is caused by the application of pressure and heat during the rock’s formation. This process causes the minerals within the rock to realign themselves, forming a layered or banded appearance.
Foliated rocks are types of metamorphic rocks that have been subjected to heat and pressure, causing them to develop a layered or banded appearance. Slate, schist, and gneiss are examples of foliated rocks. Slate is a fine-grained rock that is formed from the metamorphism of shale, while schist is a coarse-grained rock that is formed from the metamorphism of mudstone or shale. Gneiss is a high-grade metamorphic rock that is formed from the metamorphism of granite or other igneous rocks.
Non-foliated rocks, on the other hand, lack the layered or banded appearance of foliated rocks. They are typically formed from a single mineral or mineral type. Marble and quartzite are examples of non-foliated rocks. Marble is a metamorphic rock that is formed from the metamorphism of limestone or dolomite, while quartzite is formed from the metamorphism of sandstone.
Rockhounds most often find metamorphic rocks in areas where there are high amounts of geological activity such as mountain ranges or areas with high levels of tectonic activity.
Examples of metamorphic rocks
Examples of metamorphic rocks include:
- Gneiss
- Marble
- Quartzite
- Schist
- Slate
- Amphibolite
- Blueschist
- Eclogite
- Granulite
- Hornfels
- Migmatite
- Phyllite
- Skarn
- Soapstone
- Anthracite
- Serpentinite
Step 2 – Determine the rock’s basic characteristics
Once you’ve determined the type of rock, check the rock’s grain size and hardness. These are the most important characteristics of rock identification for a rockhound.
Rock grain size
If the grains are visible without a magnifier, it’s considered coarse. They are considered fine if they are small enough to require a magnifier to see them. The table below has the grain size names and measurements to use when identifying a rock type.
Size | Diameter (mm) | Grain size name |
Larger | Boulder | |
256 | Cobble | |
64 | Pebble | |
2 | Sand | |
1/16 | Silt | |
Smaller | 1/256 | Clay (indistinguishable) |
Rock hardness
Rock hardness is measured using the Mohs scale and refers to the hardness of the minerals within the rock. Hard rock scratches glass and steel, usually signifying the minerals quartz or feldspar are present. Soft rock doesn’t scratch steel but will scratch fingernails, while very soft rock won’t even scratch fingernails.
Rockhounds use fingernails, coins, knife blades, steel files, glass, and other rocks and minerals to test the hardness of a rock or mineral. The table below illustrates the Mohs’ hardness scale (the numerical value in column 2), how to determine the Mohs’ hardness value, and an example of the mineral that represents the Mohs’ hardness.
Mineral | Hardness | How to Test |
Talc | 1 | Can be scratched by a fingernail |
Gypsum | 2 | Can be scratched by a fingernail, or scratched by a copper coin |
Calcite | 3 | Can be scratched by a copper coin, or scratched by a steel knife |
Fluorite | 4 | Can be scratched by a steel knife, or scratched by a small piece of quartz |
Apatite | 5 | Can be scratched by a small piece of quartz, or scratched by a steel file |
Orthoclase feldspar | 6 | Can be scratched by a steel file, or scratched by a piece of window glass |
Quartz | 7 | Can scratch window glass |
Topaz | 8 | Can scratch quartz |
Corundum | 9 | Can scratch topaz |
Diamond | 10 | Can scratch all the minerals above |
Step 3 – use the appropriate rock identification chart for your rock type
Now that you’ve determined the rock type, grain size, and hardness, you are ready to begin pinpointing the specific rock or mineral. Use the rockhound rock identification charts below to help you identify your rock. Start with the table for your rock’s specific type. Pay attention to the color and composition of the rock’s grain. Test its hardness. Start in the left column of the appropriate table and work your way across.
Igneous Rock Identification
Hardness | Grain Size | Usual Color | Other | Composition | Rock Type |
---|---|---|---|---|---|
5-5.5 | fine | dark | glassy appearance | lava glass | Obsidian |
5-6 | fine | light | many small bubbles | lava froth from sticky lava | Pumice |
5-6 | fine | dark | many large bubbles | lava froth from fluid lava | Scoria |
6-7 | fine or mixed | light | contains quartz | high-silica lava | Felsite |
7 | fine or mixed | medium | between felsite and basalt | medium-silica lava | Andesite |
6 | fine or mixed | dark | has no quartz | low-silica lava | Basalt |
5-7 | mixed | any color | large grains in fine-grained matrix | large grains of feldspar, quartz, pyroxene or olivine | Porphyry |
6-7 | coarse | light | wide range of color and grain size | feldspar and quartz with minor mica, amphibole or pyroxene | Granite |
6-7 | coarse | light | like granite but without quartz | feldspar with minor mica, amphibole or pyroxene | Syenite |
5-7 | coarse | light to medium | little or no https://www.thoughtco.com/gallery-of-feldspars-4122850 | plagioclase and quartz with dark minerals | Tonalite |
5-6 | coarse | medium to dark | little or no quartz | low-calcium plagioclase and dark minerals | Diorite |
6-7 | coarse | medium to dark | no quartz; may have olivine | high-calcium plagioclase and dark minerals | Gabbro |
5.5-6.5 | coarse | dark | dense; always has olivine | olivine with amphibole and/or pyroxene | Peridotite |
5-6 | coarse | dark | dense | mostly pyroxene with olivine and amphibole | Pyroxenite |
5.5-6.5 | coarse | green | dense | at least 90 percent olivine | Dunite |
6-7 | very coarse | any color | usually in small intrusive bodies | typically granitic | Pegmatite |
Sedimentary Rock Identification
Hardness | Grain Size | Composition | Other | Rock Type |
hard | coarse | clean quartz | white to brown | Sandstone |
hard | coarse | quartz and feldspar | usually very coarse | Arkose |
hard or soft | mixed | mixed sediment with rock grains and clay | gray or dark and “dirty” | Wacke/Graywacke |
hard or soft | mixed | mixed rocks and sediment | round rocks in finer sediment matrix | Conglomerate |
hard orsoft | mixed | mixed rocks and sediment | sharp pieces in finer sediment matrix | Breccia |
hard | fine | very fine sand; no clay | feels gritty on teeth | Siltstone |
hard | fine | chalcedony | no fizzing with acid | Chert |
soft | fine | clay minerals | splits in layers | Shale |
soft | fine | carbon | black; burns with tarry smoke | Coal |
soft | fine | calcite | https://www.thoughtco.com/what-is-the-acid-test-4123174 | Limestone |
soft | coarse or fine | dolomite | no fizzing with acid unless powdered | Dolomite rock |
soft | coarse | fossil shells | mostly pieces | Coquina |
very soft | coarse | halite | salt taste | Rock Salt |
very soft | coarse | gypsum | white, tan or pink | Rock Gypsum |
Metamorphic Rock Identification
Hardness | Foliation | Grain Size | Usual Color | Other | Rock Type |
---|---|---|---|---|---|
1 | foliated | fine | light | very soft; greasy feel | Soapstone |
2-2.5 | foliated | fine | dark | soft; strong cleavage | Slate |
2.5-4 | nonfoliated | fine | dark | soft; massive structure | Argillite |
3-4 | foliated | fine | dark | shiny; crinkly foliation | Phyllite |
5-7 | foliated | coarse | mixed dark and light | crushed and stretched fabric; deformed large crystals | Mylonite |
6-7 | foliated | coarse | mixed dark and light | wrinkled foliation; often has large crystals | Schist |
6-7 | foliated | coarse | mixed | banded | Gneiss |
6-7 | foliated | coarse | mixed | distorted “melted” layers | Migmatite |
6 | foliated | coarse | dark | mostly hornblende | Amphibolite |
2.5-5.5 | nonfoliated | fine | greenish | soft; shiny, mottled surface | Serpentinite |
2.5-3 | nonfoliated | fine or coarse | dark | dull and opaque colors, found near intrusions | Hornfels |
6-7 | nonfoliated | coarse | red and green | dense; garnet and pyroxene | Eclogite |
3-4 | nonfoliated | coarse | light | soft; calcite or dolomite by the acid test | Marble |
7 | nonfoliated | coarse | light | quartz (no fizzing with acid) | Quartzite |
A complete list of rocks by rock type
Below is an extensive list of rocks by rock type. A dedicated rockhound will become familiar with all the rocks in the lists. Once familiar with these rocks, a rockhound is able to easily and quickly identify a rock just by looking at it.
List of igneous rocks
Igneous rocks | Rock description | Mohr’s hardness | Distinguishing properties |
Adakite | Volcanic rock type | 6-7 | High Sr/Y ratio |
Andesite | Type of volcanic rock | 5-6 | Fine-grained, porphyritic texture |
Alkali feldspar granite | Type of igneous rock rich in alkali feldspar | 5-6 | Light-colored, coarse-grained texture |
Anorthosite | Mafic intrusive igneous rock composed predominantly of plagioclase | 6-7 | Light-colored, coarse-grained texture |
Aplite | Fine-grained intrusive igneous rock type similar to granite | 5-6 | Light-colored, fine-grained texture |
Basalt | Magnesium- and iron-rich extrusive igneous rock | 5-6 | Dark-colored, fine-grained texture |
ʻAʻā | Molten rock expelled by a volcano during an eruption – Basaltic lava with a crumpled appearance | 5-6 | Rough, sharp texture |
Pāhoehoe | Molten rock expelled by a volcano during an eruption – Basaltic lava with a flowing, often ropy appearance | 5-6 | Smooth, ropy texture |
Basaltic trachyandesite | 5-6 | Intermediate between basalt and andesite | |
Mugearite | Volcanic rock type | 5-6 | Alkali basalt |
Shoshonite | Potassium-rich variety of basaltic trachyandesite | 5-6 | High K2O/Na2O ratio |
Basanite | Type of volcanic rock | 5-6 | Dark-colored, fine-grained texture |
Blairmorite | Rare porphyritic volcanic rock | 4-5 | Porphyritic texture, high Al2O3 content |
Boninite | Ultramafic extrusive rock high in both magnesium and silica | 5-6 | High MgO content |
Carbonatite | Igneous rock with more than 50% carbonate minerals | 3.5-4 | Contains more than 50% carbonate minerals |
Charnockite | Type of granite containing orthopyroxene | 6-7 | Coarse-grained texture, high Na2O/K2O ratio |
Enderbite | Igneous rock of the charnockite series | 6-7 | Coarse-grained texture, high Na2O/K2O ratio |
Dacite | Volcanic rock intermediate in composition between andesite and rhyolite | 6-7 | Intermediate between andesite and rhyolite |
Diabase, also known as dolerite | Type of igneous rock | 6 | Dark-colored, fine-grained texture |
Diorite | Igneous rock type | 6-7 | Coarse-grained texture |
Napoleonite, also known as corsite | Variety of diorite with orbicular structure | 6-7 | Orbicular structure |
Dunite | Ultramafic and ultrabasic rock from Earth’s mantle which is made of the mineral olivine | 5-6 | High olivine content |
Essexite | Igneous rock type | 6-7 | Intermediate between diorite and granite |
Foidolite | Igneous rock rich in feldspathoid minerals | 5-6 | Rich in feldspathoid minerals |
Gabbro | Coarse-grained mafic intrusive rock | 6-7 | Coarse-grained texture |
Granite | Common type of intrusive, felsic, igneous rock with a granular structure | 6-7 | Coarse-grained texture |
Granodiorite | Type of coarse-grained intrusive igneous rock | 6-7 | Coarse-grained texture |
Granophyre | Subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths | – | Contains quartz and alkali feldspar |
Harzburgite | Ultramafic mantle rock | 5-6 | Coarse-grained texture |
Hornblendite | Plutonic rock consisting mainly of the amphibole hornblende | – | Consists mainly of hornblende |
Hyaloclastite | Volcaniclastic accumulation or breccia | – | – |
Icelandite | Igneous rock type | 5-6 | – |
Ignimbrite | Type of volcanic rock | 4-5 | – |
Ijolite | Igneous rock consisting essentially of nepheline and augite | – | Consists of nepheline and augite |
Kimberlite | Igneous rock which sometimes contains diamonds | – | – |
Komatiite | Ultramafic mantle-derived volcanic rock | 5-6 | – |
Lamproite | Ultrapotassic mantle-derived volcanic or subvolcanic rock | – | Ultrapotassic |
Lamprophyre | Ultrapotassic igneous rocks – An ultramafic, ultrapotassic intrusive rock dominated by mafic phenocrysts in a feldspar groundmass | – | Ultrapotassic |
Latite | Type of volcanic rock – A silica-undersaturated form of andesite | 5-6 | Silica-undersaturated |
Lherzolite | An ultramafic and ultrabasic rock that is composed of olivine and pyroxene – An ultramafic rock, essentially a peridotite | 5-6 | Composed of olivine and pyroxene |
Monzogranite | A silica-undersaturated granite with <5% normative quartz | 6-7 | Silica-undersaturated |
Monzonite | Igneous intrusive rock with low quartz and equal plagioclase and alkali feldspar – a plutonic rock with <5% normative quartz | 6-7 | Low quartz |
Nepheline syenite | Holocrystalline plutonic rock – A silica-undersaturated plutonic rock of nepheline and alkali feldspar | 5-6 | Silica-undersaturated |
Nephelinite | Igneous rock made up almost entirely of nepheline and clinopyroxene – A silica-undersaturated plutonic rock with >90% nepheline | – | Silica-undersaturated |
Norite | Igneous rock – A hypersthene-bearing gabbro | 6-7 | Hypersthene-bearing |
Obsidian | Naturally occurring volcanic glass | 5-5.5 | – |
Pegmatite | Igneous rock with very large interlocked crystals | – | Very large, interlocked crystals |
Peridotite | Coarse-grained ultramafic igneous rock type | 5.5-6 | Dark green color, high density |
Phonolite | Uncommon extrusive rock – A silica-undersaturated volcanic rock; essentially similar to nepheline syenite | 5-6 | Fine-grained, light-colored, contains feldspathoids |
Phonotephrite | Strongly alkaline volcanic rock with a composition between phonolite and tephrite – A volcanic rock with a composition between phonolite and tephrite | 5-6 | Fine-grained, porphyritic texture |
Picrite | Variety of high-magnesium basalt that is very rich in the mineral olivine – An olivine-bearing basalt | 5-6 | Dark green color, high density, abundant olivine |
Porphyry | Textural form of igneous rock with large-grained crystals in a fine matrix | 6-7 | Porphyritic texture, often contains quartz and feldspar |
Pumice | Light colored highly vesicular volcanic rock | 5-6 | Very low density, often floats on water |
Pyroxenite | Igneous rock – a coarse-grained plutonic rock composed of >90% pyroxene | 5.5-6 | Dark color, abundant pyroxene |
Quartz diorite | Igneous, plutonic rock – A diorite with >5% modal quartz | 6-7 | Light color, abundant quartz |
Quartz monzonite | Type of igneous rock – An intermediate plutonic rock, essentially a monzonite with 5–10% modal quartz | 6-7 | Light color, abundant quartz and feldspar |
Quartzolite | Extremely rare igneous rock made mostly of quartz – An intrusive rock composed mostly of quartz | 7 | White or light-colored, composed mostly of quartz |
Rhyodacite | Volcanic rock rich in silica and low in alkali metal oxides – A felsic volcanic rock which is intermediate between a rhyolite and a dacite | 6-7 | Porphyritic texture, light color |
Rhyolite | Igneous, volcanic rock, of felsic (silica-rich) composition | 6 | Fine-grained, often contains quartz and feldspar |
Comendite | Hard, peralkaline igneous rock, a type of light blue-gray rhyolite | 6 | Light blue-gray color, peralkaline composition |
Pantellerite | Peralkaline rhyolite type of volcanic rock | 6 | Light-colored, peralkaline composition |
Scoria | Dark vesicular volcanic rock | 5-6 | Porous texture, dark color |
Shonkinite | Intrusive igneous rock – a plutonic rock | 5-6 | Dark color, composed of feldspar and pyroxene |
Sovite | Igneous rock – A coarse-grained carbonatite rock | 3-4 | Light color, composed of carbonate minerals |
Syenite | Intrusive igneous rock – A plutonic rock dominated by orthoclase feldspar; a type of granitoid | 5-6 | Light color, composed of feldspar and mafic minerals |
Tachylyte | Form of basaltic volcanic glass – Essentially a basaltic glass | 5.5-6 | Glassy texture, dark color |
Tephriphonolite | Type of igneous rock – A volcanic rock with a composition between phonotephrite and phonolite | 5-6 | Fine-grained, contains feldspathoids |
Tephrite | Igneous, volcanic rock – A silica-undersaturated volcanic rock | 5-6 | Low density, feldspathoid-bearing |
Tonalite | Igneous rock – A plagioclase-dominant granitoid | 6-7 | Plagioclase-dominant, coarse-grained |
Trachyandesite | Extrusive igneous rock – An alkaline intermediate volcanic rock | 5-6 | Porphyritic, fine-grained |
Benmoreite | Volcanic rock type – sodic trachyandesite | 5-6 | Porphyritic, fine-grained |
Trachybasalt | Volcanic rock – A volcanic rock with a composition between basalt and trachyte | 5-6 | Porphyritic, fine-grained |
Hawaiite | Volcanic rock – a sodic type of trachybasalt, typically formed by ocean island (hot spot) volcanism | 5-6 | Porphyritic, fine-grained |
Trachyte | Extrusive igneous rock – A silica-undersaturated volcanic rock; essentially a feldspathoid-bearing rhyolite | 5-6 | Porphyritic, feldspathoid-bearing |
Troctolite | Igneous rock – A plutonic ultramafic rock containing olivine, pyroxene and plagioclase | 5-6 | Coarse-grained, olivine-rich |
Trondhjemite | Light-colored intrusive igneous rock – A form of tonalite where plagioclase-group feldspar is oligoclase | 6-7 | Plagioclase-dominant, coarse-grained |
Tuff | Rock consolidated from volcanic ash | 1-2 | Porous, lightweight |
Websterite | Ultramafic and ultrabasic rock – A type of pyroxenite, composed of clinoproxene and orthopyroxene | 5-6 | Coarse-grained, ultramafic |
Wehrlite | Ultramafic rock – An ultramafic plutonic or cumulate rock, a type of peridotite, composed of olivine and clinopyroxene | 5-6 | Coarse-grained, ultramafic |
List of sedimentary rocks
Sedimentary rocks | Description | Mohr’s Hardness Rating | Distinguishing Properties |
Argillite | Sedimentary rock, mostly of indurated clay particles | 1-2 | Fine-grained, dark gray to black-colored rock |
Arkose | Type of sandstone containing at least 25% feldspar | 6-7 | Coarse-grained, reddish-brown colored rock |
Banded iron formation | Distinctive layered units of iron-rich sedimentary rock that are almost always of Precambrian age | 5.5-6.5 | Alternating bands of iron oxide and chert |
Breccia | Rock composed of broken fragments cemented by a matrix | 2.5-3 | Angular fragments of different rocks and minerals, cemented by finer-grained material |
Calcarenite | Type of limestone that is composed predominantly of sand-size grains | 3 | Light-colored, composed mainly of sand-sized grains |
Chalk | Soft, white, porous sedimentary rock made of calcium carbonate | 1-2 | Soft and crumbly, white or gray-colored rock |
Chert | Hard, fine-grained sedimentary rock composed of cryptocrystalline silica | 6.5-7 | Hard, brittle, usually gray-colored rock |
Claystone | Clastic sedimentary rock composed primarily of clay-sized particles | 1-2 | Fine-grained, typically gray-colored rock |
Coal | Combustible sedimentary rock composed primarily of carbon | 1-2 | Hard, typically gray or black-colored rock |
Conglomerate | Coarse-grained sedimentary rock composed mostly of rounded to sub-angular fragments | 6-7 | Rounded or angular fragments of different sizes and types of rocks, cemented by finer-grained material |
Coquina | Sedimentary rock that is composed mostly of fragments of shells | 3 | Composed of broken shells and shell fragments, typically light-colored |
Diamictite | Type of sedimentary rock | 2.5-4 | Unsorted mixture of different sizes and types of rocks, with a matrix of finer-grained material |
Diatomite | Soft, siliceous sedimentary rock that is easily crumbled | 5.5-6.5 | Light-colored, composed mainly of the silica shells of diatoms |
Dolomite (rock), also known as Dolostone | Sedimentary carbonate rock that contains a high percentage of the mineral dolomite | 3.5-4 | Light-colored, typically granular or crystalline |
Evaporite | Water-soluble mineral deposit formed by evaporation from an aqueous solution | 2.5-4 | Composed of minerals such as halite, gypsum, or anhydrite, formed by evaporation of water |
Flint | Cryptocrystalline form of the mineral quartz | 7 | Coarse-grained, typically gray-colored rock |
Geyserite | Form of opaline silica that is often found around hot springs and geysers | 5.5-6.5 | Formed from the precipitation of dissolved silica around hot springs and geysers |
Greywacke | Hard, dark sandstone with poorly sorted angular grains in a compact, clay-fine matrix | 6-7 | Composed of angular fragments of different types of rocks, with a matrix of finer-grained material |
Gritstone | Hard, coarse-grained, siliceous sandstone | 6-7 | Product of rock weathering in wet tropical climate rich in iron and aluminum |
Itacolumite | Porous sandstone known for flexibility | 2.5-3 | Composed of sand-sized grains that are loosely cemented together, allowing the rock to be flexible |
Jaspillite | Banded mixture of hematite and quartz | 6-7 | Alternating bands of red-colored hematite and gray-colored quartz |
Laterite | Typically gray-colored, composed of a mixture of calcium carbonate and clay minerals | 3-3.5 | Reddish-brown colored rock, formed from the weathering of other rocks in tropical regions |
Lignite | Soft, brown, combustible, sedimentary rock | 1-2 | Light-colored, composed of organic material such as peat |
Limestone | Sedimentary rocks made of calcium carbonate | 3-4 | Light-colored, composed mainly of calcium carbonate |
Marl | Lime-rich mud or mudstone which contains variable amounts of clays and silt | 2-4 | Typically gray colored, composed of a mixture of calcium carbonate and clay minerals |
Mudstone | Typically gray-colored, composed of phosphate minerals such as apatite | 1.5-3 | Fine-grained, typically gray colored rock |
Oil shale | Organic-rich fine-grained sedimentary rock containing kerogen | 2-4 | Fine-grained sedimentary rock whose original constituents were clays or mud |
Oolite | Sedimentary rock formed from ooids | 3.5-4 | Composed of small, spherical grains called ooids, typically light-colored |
Phosphorite | Sedimentary rock containing high amounts of phosphate minerals | 3.5-4 | Fine-grained, typically gray-colored rock that splits into thin layers |
Sandstone | Clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments | 6-7 | Fine-grained, typically gray-colored rock |
Shale | Fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite | 3-4 | Coarse-grained, typically tan or brown-colored rock |
Siltstone | Sedimentary rock composed mainly of silt-sized particles | 6-7 | Coarse-grained, typically gray-colored rock composed of a mixture of different types of rocks and minerals |
Travertine | Sedimentary rock, mostly of calcium carbonate, formed from the accumulation of mineral deposits from hot springs or streams | 4-5 | Light-colored, formed from the precipitation of calcium carbonate from water |
Tufa | Porous limestone rock formed by precipitation of carbonate minerals from ambient temperature water bodies | 5-7 | Porous, light-colored rock formed from the precipitation of calcium carbonate from water |
Turbidite | Sedimentary deposit formed when gravity-driven downslope movement of fluidized, sediment-laden flow overcomes frictional forces and is deposited – A graded sequence of sedimentary rocks | 6-7 | Coarse-grained, typically gray colored rock composed of a mixture of different types of rocks and minerals |
Wackestone | Mud-supported carbonate rock that contains greater than 10% grains | – | – |
List of metamorphic rocks
Metamorphic Rocks | Description | Mohr’s Hardness Rating | Distinguishing Properties |
Anthracite | Hard, compact variety of coal | 2.5-3.5 | Shiny black, breaks with conchoidal fracture |
Amphibolite | A metamorphic rock containing mainly amphibole and plagioclase | 6-7 | Class of high-grade medium to coarse-grained metamorphic rocks |
Blueschist | Type of metavolcanic rock | 5-6 | Blue-green color, often with visible crystals |
Cataclasite | Rock found at geological faults – A rock formed by faulting | 2.5-6 | Fragmented texture, formed by faulting |
Eclogite | A dense metamorphic rock formed under high pressure | 6.5-7.5 | Green and red coloration, often with visible crystals |
Gneiss | Common high-grade metamorphic rock | 6-7 | Banded appearance, with alternating layers of different minerals |
Granulite | Type of foliated metamorphic rock – A low-grade metamorphic rock composed mostly of micaceous minerals | 7-8 | Coarse-grained, with visible crystals |
Greenschist | A mafic metamorphic rock dominated by green amphiboles | 5-6 | Green coloration, often with visible crystals |
Hornfels | Series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses. | 6-7 | Fine-grained, often with a dull luster |
Calcflinta | A type of hornfels found in the Scottish Highlands | – | Fine-grained, dark-colored rock |
Litchfieldite | Nepheline syenite gneiss | – | Light-colored, with visible crystals |
Marble | Type of rock – a metamorphosed limestone | 3-4 | Light-colored, often with visible veins |
Migmatite | Mixture of metamorphic rock and igneous rock | – | Banded appearance, with layers of different minerals |
Mylonite | A metamorphic rock formed by shearing | 4-6 | Fine-grained, often with a foliated texture |
Metapelite | A metamorphic rock with a protolith of clay-rich (siltstone) sedimentary rock | – | Fine-grained, often with visible foliation |
Metapsammite | A metamorphic rock with a protolith of quartz-rich (sandstone) sedimentary rock | – | Coarse-grained, often with visible crystals |
Phyllite | Metamorphic rock – A low-grade metamorphic rock formed from shale or silts | 1-2 | Fine-grained, often with visible foliation |
Pseudotachylite | Glassy, or very fine-grained, rock type – A glass formed by melting within a fault via friction | – | Dark-colored, glassy texture |
Quartzite | Hard, non-foliated metamorphic rock which was originally pure quartz sandstone – A metamorphosed sandstone typically composed of >95% quartz | 7 | Shiny appearance, glassy surface, with few visible minerals |
Schist | Easily split medium-grained metamorphic rock | 3.5-4 | Foliated texture, visible minerals, medium to coarse-grained |
Serpentinite | Rock formed by hydration and metamorphic transformation of olivine | 2.5-4 | Dark green color, often with a scaly texture, can be easily carved |
Skarn | Hard, coarse-grained, hydrothermally altered metamorphic rocks | 3.5-4 | Banded appearance, with alternating layers of different minerals, often with visible crystals |
Slate | A metamorphosed ultramafic rock with talc as an essential constituent, similar to a serpentinite | 2.5-4 | Fine-grained, often with visible foliation, splits into thin sheets |
Suevite | Rock consisting partly of melted material formed during an impact event – A rock formed by partial melting during a meteorite impact | – | Brecciated texture, with clasts of various sizes and shapes, often with melt veins |
Talc carbonate | A high-pressure metamorphic rock containing talc and kyanite | ||
Soapstone – Talc-bearing metamorphic rock – Essentially a talc schist | – | Soft texture, can be easily scratched with a fingernail, often with a soapy feel | |
Tectonite | Rock type – A rock whose fabric reflects the history of its deformation | – | Foliated texture, visible minerals, medium to coarse-grained |
Whiteschist | A high pressure metamorphic rock containing talc and kyanite | – | White color, often with a banded appearance, visible minerals |
List of Misc rock varieties
Igneous rocks | Description | Mohr’s Hardness Rating | Distinguishing Properties |
Adamellite | Type of igneous rock – A variety of quartz monzonite | 5.5-6.5 | Light-colored, often with visible veins |
Appinite | A group of varieties of lamprophyre, mostly rich in hornblende | 5-6 | Fine-grained, often with a dark color |
Aphanite | Igneous rock composed of very small crystals invisible to the naked eye | 5-6 | Uniform texture, often with a porphyritic structure |
Borolanite | Variety of nepheline syenite from Loch Borralan, Scotland – A variety of nepheline syenite from Loch Borralan, Scotland | 5.5-6.5 | Coarse-grained, often with visible crystals |
Blue Granite | Variety of monzonite, an igneous rock | 6-7 | Visible crystals, often with a blue-gray color |
Epidosite | Hydrothermally altered epidote- and quartz-bearing rock | 2.5-3.5 | Green color, often with visible quartz veins |
Felsite | Very fine-grained volcanic rock that sometimes contains larger crystals | 6-7 | Prone to weathering, often with a reddish color |
Flint | Cryptocrystalline form of the mineral quartz | 7 | Conchoidal fracture, often used to make tools |
Ganister | Hard, fine-grained quartzose sandstone, or orthoquartzite | 7 | High silica content, often used in refractories |
Gossan | Intensely oxidized, weathered or decomposed rock | 4-5 | Reddish-brown color, often with visible iron oxides |
Hyaloclastite | Volcaniclastic accumulation or breccia | – | Glassy texture, formed by volcanic activity |
Ijolite | Igneous rock consisting essentially of nepheline and augite | 5-6 | Coarse-grained, often with visible crystals |
Jadeitite | Metamorphic rock found in blueschist-grade metamorphic terranes | 6.5-7 | Green color, often with visible jadeite minerals |
Jasperoid | A hematite-silica metasomatite analogous to a skarn | – | Reddish-brown color, often with visible hematite |
Kenyte | Type of igneous rock – A variety of phonolite, first found on Mount Kenya | 5-6 | Fine-grained, often with a porphyritic structure |
Lapis lazuli | Metamorphic rock containing lazurite, prized for its intense blue color – A rock composed of lazurite and other minerals | 5-5.5 | Blue color, often with visible veins |
Larvikite | Variety of monzonite, an igneous rock | 6-7 | Visible crystals, often with a blue-gray color |
Litchfieldite | A metamorphosed nepheline syenite occurrence near Litchfield, Maine | – | Light-colored, often with visible crystals |
Llanite | Type of mineral – A hypabyssal rhyolite with microcline and blue quartz phenocrysts from the Llano Uplift in Texas | – | Visible blue quartz phenocrysts |
Luxullianite | Rare type of granite | 6-7 | Coarse-grained, often with visible crystals |
Mangerite | Plutonic intrusive igneous rock, that is essentially a hypersthene-bearing monzonite | 6-7 | Light-colored, often with visible veins |
Minette | A variety of lamprophyre | 5-6 | Fine-grained, often with a dark color |
Novaculite | A type of chert found in Oklahoma, Arkansas, and Texas | – | – |
Pietersite | – | – | Breccia rock of hawk’s eyes and tiger’s eyes |
Pyrolite | Theoretical rock making up the earth’s upper mantle | – | Large-grained crystals in a fine matrix, euhedral rhombic phenocrysts of feldspar |
Rapakivi granite | Type of igneous rock | – | – |
Rhomb porphyry | Textural form of igneous rock – A type of latite with rhombic phenocrysts of feldspar | 6-7 | Large grained crystals in a fine matrix, euhedral rhombic phenocrysts of feldspar |
Rodingite | Metamorphic rock – A mafic rock metasomatized by serpentinization fluids | 5-6 | Metasomatized by serpentinization fluids |
Shonkinite | Intrusive igneous rock – melitilic and kalsititic rocks | 5-6 | Fine-grained, often with a porphyritic structure |
Taconite | Variety of iron-bearing sedimentary rock | – | – |
Tachylite | Form of basaltic volcanic glass | 5.5-6 | Glassy texture |
Teschenite | A silica undersaturated, analcime bearing gabbro | – | Analcime bearing |
Theralite | Igneous rock – A nepheline gabbro | 5-6 | – |
Unakite | Metamorphic rock – An altered granite | – | – |
Variolite | Igneous rocks which contain varioles | – | Contains varioles |
Vogesite | Ultrapotassic igneous rocks – A variety of lamprophyre | – | Coarse-grained, often with visible crystals |
Wad | Porous secondary manganese oxyhydroxide – A rock rich in manganese oxide or manganese hydroxide | – | Rich in manganese oxide or manganese hydroxide |
Additional resources for rockhounds
Rock identification flowcharts
A rockhound can use flowcharts like the ones below to move through the rock-identification steps. Start with the boxes on the left and work your way to the right. You can use these charts when rockhounding to quickly identify a rock or rock type without conducting detailed tests.
The most desirable (and valuable) rocks, minerals and gems by state
Below is a table of all 50 states and the most valuable rocks, minerals, and gems that are commonly found by rockhounds in each state.
State | Rocks, Minerals, and Gems |
---|---|
Alabama | Hematite, Quartz, Pyrite, Limestone, Marble, Barite, Coal |
Alaska | Gold, Quartz, Tourmaline, Garnets, Zinc, Lead, Silver, Platinum, Diamonds |
Arizona | Turquoise, Copper, Amethyst, Garnets, Silver, Gold, Diamonds, Petrified Wood |
Arkansas | Diamonds, Quartz, Bauxite, Wavellite, Marcasite, Magnetite, Barite, Galena |
California | Gold, Jade, Serpentine, Tourmaline, Garnets, Topaz, Aquamarine, Benitoite |
Colorado | Rhodochrosite, Amazonite, Aquamarine, Fluorite, Topaz, Garnets, Quartz, Amethyst |
Connecticut | Garnet, Beryl, Topaz, Danburite, Sulfides, Wollastonite, Staurolite |
Delaware | Sillimanite, Zinc, Kaolin, Iron, Beryl, Talc, Feldspar |
Florida | Agatized Coral, Calcite, Barite, Phosphate, Apatite, Celestine, Selenite |
Georgia | Staurolite, Kyanite, Quartz, Beryl, Galena, Pyrite, Limonite, Magnesite |
Hawaii | Black Coral, Olivine, Quartz, Tourmaline, Jasper, Agate, Halite |
Idaho | Star Garnet, Opal, Jade, Gold, Silver, Galena, Pyrite, Sapphires |
Illinois | Fluorite, Galena, Calcite, Barite, Sphalerite, Celestine, Marcasite |
Indiana | Calcite, Selenite, Celestine, Pyrite, Dolomite, Quartz, Sphalerite |
Iowa | Geodes, Calcite, Barite, Celestine, Pyrite, Sphalerite, Marcasite |
Kansas | Barite, Calcite, Galena, Fluorite, Selenite, Celestine, Halite |
Kentucky | Fluorite, Barite, Calcite, Galena, Sphalerite, Marcasite, Pyrite |
Louisiana | Agate, Limestone, Sulfur, Barite, Halite, Gypsum, Petroleum |
Maine | Tourmaline, Beryl, Quartz, Mica, Feldspar, Gems, Pegmatites |
Maryland | Agate, Selenite, Chalcedony, Native Copper, Serpentine, Jasper, Staurolite |
Massachusetts | Rhodonite, Babingtonite, Tourmaline, Beryl, Topaz, Mica, Feldspar |
Michigan | Copper, Agate, Greenstone, Datolite, Chlorastrolite, Calcite, Celestine |
Minnesota | Lake Superior Agate, Calcite, Thomsonite, Sphalerite, Galena, Pyrite, Quartz |
Mississippi | Magnesite, Sphalerite, Barite, Fluorite, Calcite, Celestine, Galena |
Missouri | Galena, Calcite, Barite, Sphalerite, Marcasite, Pyrite, Celestine |
Montana | Sapphires, Garnet, Agate, Quartz, Gold, Silver, Copper, Platinum |
Nebraska | Prairie Agate, Calcite, Rhodochrosite, Chalcedony, Quartz, Selenite, Pyrite |
Nevada | Turquoise, Opal, Garnet, Silver, Gold, Copper, Lead, Barite |
New Hampshire | Smoky Quartz, Beryl, Topaz, Tourmaline, Mica, Feldspar, Pegmatites |
New Jersey | Fluorite, Magnetite, Zinc, Andradite Garnet, Prehnite, Pyrite, Franklinite |
New Mexico | Turquoise, Copper, Geodes, Agate, Fluorite, Calcite, Barite |
New York | Garnet, Herkimer Diamonds, Calcite, Celestine, Sphalerite, Galena, Dolomite |
North Carolina | Emerald, Ruby, Sapphire, Aquamarine, Quartz, Feldspar, Mica, Spodumene |
North Dakota | Petrified Wood, Agate, Calcite, Gypsum, Selenite, Barite, Celestine |
Ohio | Fluorite, Celestine, Calcite, Sphalerite, Galena, Pyrite, Dolomite |
Oklahoma | Barite, Quartz, Selenite, Galena, Calcite, Sphalerite, Pyrite |
Oregon | Sunstone, Opal, Jasper, Agate, Thundereggs, Zeolites, Obsidian |
Pennsylvania | Quartz, Calcite, Pyrite, Sphalerite, Galena, Celestine, Geodes |
Rhode Island | Bowenite, Quartz, Pyrite, Selenite, Calcite, Garnet, Beryl |
South Carolina | Amethyst, Beryl, Kyanite, Aquamarine, Quartz, Feldspar, Mica |
South Dakota | Fairburn Agate, Rose Quartz, Petrified Wood, Calcite, Pyrite, Barite, Galena |
Tennessee | Agate, Fluorite, Calcite, Pyrite, Sphalerite, Galena, Barite |
Texas | Topaz, Fluorite, Geodes, Agate, Calcite, Selenite, Barite |
Utah | Topaz, Copper, Beryllium, Garnet, Agate, Fluorite, Calcite |
Vermont | Grossular Garnet, Beryl, Quartz, Talc, Graphite, Garnet, Mica |
Virginia | Kyanite, Amazonite, Garnet, Pyrite, Sulfides, Staurolite, Epidote |
Washington | Petrified Wood, Agate, Jasper, Garnet, Topaz, Quartz, Pyrite |
West Virginia | Coal, Cassiterite, Quartz, Celestine, Sphalerite, Galena, Pyrite |
Wisconsin | Galena, Agate, Calcite, Sphalerite, Celestine, Pyrite, Marcasite |
Wyoming | Nephrite Jade, Jadeite, Agate, Sapphires, Garnet, Gold, Platinum |
Image Credits
In-Article Image Credits
Rocky side of a mountain via Wikimedia Commons with usage type - GNU Free. August 23, 2006Simplified rock identification and origin flowchart via Purdue University with usage type - Editorial use (Fair Use)
Mineral identification flowchart via Purdue University with usage type - Editorial use (Fair Use)
Rock identification flowchart via Purdue University with usage type - Editorial use (Fair Use)
Examples of Metamorphic rocks via American Museum of Science and History with usage type - Editorial use (Fair Use)
Examples of Sedimentary rocks via American Museum of Science and History with usage type - Editorial use (Fair Use)
Examples of igneous rocks via American Museum of Science and History with usage type - Editorial use (Fair Use)
Featured Image Credit
Rocky side of a mountain via Wikimedia Commons with usage type - GNU Free. August 23, 2006