How to identify rocks

Updated October 20, 2026. Based on the USGS, the National Park Service and the Mineralogical Society of America; sources are at the end.

Texture first, then simple tests

Start by looking at the texture: big crystals, fine grains, glass, bubbles or layers tell you which of the three rock families you have. Then use simple tests on the minerals in it: luster, the color of its powder on a streak plate, and hardness against a fingernail, a penny, a knife and glass. A magnet and a drop of vinegar help with some. Color alone is a weak clue, and a photo is rarely enough.

Rock or mineral: A rock is made of one or more minerals
For a rock: Look at grains, crystals, layers and bubbles
For a mineral: Luster first, then streak and hardness
Not enough: Color alone, or a photo

The three rock families

Family	What it looks like	Examples
Igneous, cooled slowly underground	Coarse grained, with crystals you can see	Granite, diorite, gabbro, peridotite
Igneous, cooled quickly at the surface	Very fine grained or glassy, often bubbly	Rhyolite, andesite, basalt, obsidian
Sedimentary	Often layered; made of pieces of older rock or of once-living things	Sandstone (sand-sized grains), limestone (calcite)
Metamorphic	Older rock changed by heat, pressure or hot fluids	—

From the USGS rock-type pages.

A slab of gray dolostone with a cavity holding clear quartz crystals — Rock and mineral together: clear quartz crystals, a mineral, in gray dolostone, a rock. From Middleville, New York. Photo: James St. John, CC BY 2.0.

Step by step

Rock or mineral? A rock is an aggregate of one or more minerals, such as quartz, feldspar, mica or calcite.
For a rock, check the texture. Large interlocking crystals suggest an igneous rock that cooled slowly, like granite. Very fine, glassy or bubbly rock suggests lava, like basalt or obsidian. Layers or sand-sized grains suggest a sedimentary rock.
For a mineral, decide the luster: metallic or not, and if not, glassy, pearly, greasy, silky, dull or earthy. The Mineralogical Society of America calls this the first critical step.
Streak: rub it on an unglazed porcelain plate and note the color of the powder. Minerals harder than about 6½ just scratch the plate.
Hardness: scratch it against the tools below, both ways.
Extra checks: a magnet for magnetite, a drop of vinegar or dilute acid for calcite, and how heavy it feels.
Think about the place. For some look-alike minerals, the Mineralogical Society of America says knowing what is found at the site is the best bet.
Still not sure? Take it to someone who can hold it: see the end of this page.

Hardness: the tool ladder

Tool	National Park Service	Mineralogical Society
Fingernail	2.5	2½
Copper penny	3.5	3
Pocket knife	5.5	5+
Window glass	5.5	5½
Steel nail or file	6.5 (nail)	6½ (file)
Quartz	7	7
Masonry drill bit	8.5	not listed

Test both ways: scratch the sample with the tool, then try to scratch the tool with the sample. Rub the mark with a finger. If it rubs off, it was powder, not a scratch. It may take a few tries.

The tools from the table, softest at the bottom. Where the two sources differ, the ladder shows both values as a range. Diagram: Capyhound.

A simple kit

An unglazed porcelain streak plate and plain white paper
A cheap pocket knife, not a Swiss Army knife
A copper penny from before 1983, or a piece of copper
A piece of steel file and a piece of quartz crystal
A small magnet; a fairly strong fridge magnet is fine
A 10x hand lens
Optional: dilute (10%) hydrochloric acid, used only with its label warnings and safety sheet. Vinegar works on calcite too, weakly.

The Mineralogical Society of America’s list.

Three quick tests

Glass: quartz is the only common mineral that easily scratches glass.
Magnet: magnetite is strongly magnetic.
Fizz: calcite fizzes in cold dilute acid, and slightly even in vinegar. Limestone is made of calcite, so it should fizz too.

Tests that mislead

Color alone: A mineral’s powder is often a different color from the piece, and luster is not color.
A Swiss Army knife: Its steel is harder than most cheap pocket knives, which throws off the scratch test.
A neodymium magnet: It attracts many ordinary rocks; use a fridge magnet.
Phone apps that measure magnetism: Useless for telling a meteorite, according to Washington University in St. Louis.
A photo: The USGS says rocks are extremely difficult to identify from photographs.

Still not sure?

The USGS says rocks and minerals must be examined in person from all sides for accurate identification. It suggests a state geological survey, a natural science museum, a college or university geology department, a rock shop, a gem and mineral club, or vendors at a gem show. To narrow it down first, try our rock identifier.

Keep a log of your finds

The Capyhound app will save each find with its photo, place and date, even with no signal.

Questions

What are the three types of rock?

Igneous rocks form when molten rock cools and hardens. Sedimentary rocks form from pieces of older rock or once-living organisms and often show layers. Metamorphic rocks are older rocks changed by heat, pressure or hot fluids.

What is the difference between a rock and a mineral?

A rock is an aggregate of one or more minerals. Common minerals include quartz, feldspar, mica and calcite.

What tools do I need to identify rocks?

An unglazed porcelain streak plate, a cheap pocket knife (not a Swiss Army knife), a copper penny from before 1983 or a piece of copper, a piece of steel file, a piece of quartz, a small magnet, a 10x hand lens and plain white paper. Dilute acid is optional, used with its safety warnings.

How do I do a scratch test?

Compare the rock with objects of known hardness: fingernail about 2.5, copper penny about 3 to 3.5, knife and glass about 5 to 5.5, steel file or nail about 6.5. Test both ways, and rub the mark to check it is a real scratch and not powder.

Does vinegar make rocks fizz?

Calcite fizzes in cold dilute acid, and slightly even in vinegar. Limestone is made of calcite, so it should react too.

Is color a good way to identify a rock?

Not on its own. The Mineralogical Society of America warns not to confuse luster with color, and a mineral’s powder, its streak, is often a different color from the piece.

Can I identify a rock from a photo?

The USGS says rocks are extremely difficult to identify from photographs. They should be examined in person, for example at a state geological survey, a museum or a university geology department.

Sources

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