Rock identifier: narrow it down with simple tests

Updated October 20, 2026. Property values from the Mineralogical Society of America’s identification key, the USGS and the National Park Service; sources are at the end.

Narrows it down, does not prove it

Answer the questions you can, and leave the rest as “Not sure”. The key compares your answers with published properties of 21 common rocks and minerals and shows the three that fit best. It narrows things down; it cannot prove what your rock is.

Covers: 21 common rocks, minerals and look-alikes
Tests: Shine, scratch, magnet, vinegar, light, pattern, streak, weight
Data from: Mineralogical Society of America, USGS, National Park Service
Photo needed: No

How to do each test

Shine: look at a fresh break, not a weathered surface. Luster is not the same as color.
Scratch: try to scratch glass with the rock, then try a steel knife and a copper coin on the rock. Test both ways, and rub the mark: powder that wipes off is not a scratch.
Magnet: a fridge magnet, not a strong neodymium one, which attracts many ordinary rocks.
Vinegar: one drop on a clean spot; calcite reacts only weakly to vinegar.
Streak: rub the rock on the unglazed back of a white tile.
Weight: compare it with an ordinary rock of the same size.

More detail on how to identify rocks.

Streak colors as in the property table below, from the Mineralogical Society of America’s key. Diagram: Capyhound.

How the key decides

Each answer is a match, a mismatch, or “no data” when the sources say nothing; no data never counts as a match.
Hardness: above glass (about 5½) counts as scratching glass; values of 5 to 6 can go either way, and about 3 sits at the copper coin.
Streak: minerals harder than the streak plate, about 6½, leave no colored line.
Magnet and vinegar: the Mineralogical Society’s key notes magnetism and fizzing where they occur, so we treat a mineral it does not mark as neither.
Weight: a specific gravity near 5, like pyrite or magnetite, counts as heavy; about 2.6 to 2.9 counts as normal.
Our inferences are marked with an asterisk in the table below.

The property table behind the key

Show the table: 21 rocks and minerals

Rock or mineral	Hardness	Streak	Luster or texture	Specific gravity	Other clues	Sources
Quartz	7	Only scratches the plate*	Vitreous	2.65	Colorless, milky, smoky to black, amethyst, rose, yellow; scratches glass easily	MSA IIIB-2 MSA streak WUSTL
Chalcedony and agate	7	Only scratches the plate*	Chalcedony waxy; agate waxy to vitreous	2.65	Agate is translucent and banded; plain chalcedony has no bands	MSA IIIB-2 NPS
Jasper	7	Only scratches the plate*	Dull	2.65	Opaque; often red, also yellow, brown, green or blue; mottled	MSA IIIB-2 NPS DOGAMI
Obsidian	—	—	Glassy texture	—	Volcanic glass that breaks to a sharp edge; flakes may be artifacts, which you may not collect	USGS DOGAMI BLM
Slag (look-alike)	—	Often none	—	—	Industrial by-product; often magnetic and bubbly; common along roads and rail lines	ASU USGS
Petrified wood	—	—	—	Denser than the original wood	Wood replaced mostly by silica: chalcedony, agate, jasper, opal or quartz; reddish from iron and manganese	NPS
Calcite	3	White	Vitreous to sub-vitreous	2.71	Fizzes in cold dilute acid, weakly in vinegar; clear pieces show a doubled image of print	MSA IIC-1
Gypsum (selenite)	2	White or none	Sub-vitreous to pearly to silky or dull	2.32	A fingernail scratches it; selenite crystals are usually clear, other kinds milky	MSA IIB-1
Feldspar: orthoclase	6 to 6½	White*	Vitreous to sub-vitreous; dull in granular masses	2.55 to 2.63	White, cream to pale yellow, pink to brownish red	MSA IIIA-1
Feldspar: plagioclase, including Oregon sunstone	6 to 6½	White*	Vitreous; labradorite can show labradorescence	2.60 to 2.76	White to gray or black; Oregon sunstone is near-colorless to pale yellow, red or green, often with copper flecks	MSA IIIA-1 GIA
Turquoise	5 to 6; massive pieces may test softer	White to pale green	Vitreous to sub-vitreous	2.6 to 2.8	Sky blue to bluish green to apple green	MSA IIIA-2
Opal	5 to 6	—	Glassy	1.9 to 2.1	Softer and lighter than quartz; precious opal shows play of color	MSA IIIA-2 NPS
Pyrite	6 to 6½	Black, may be greenish black	Metallic	5.0	Brass yellow; the most common brass-yellow metallic mineral	MSA IC
Magnetite	6	Black	Metallic or submetallic	5.18	Black; strongly magnetic	MSA IC
Hematite	5½ to 6 in crystals; as low as 1 in earthy masses	Rust red	Metallic to submetallic in crystals; earthy forms dull	4.8 to 5.3	Dark brown to steel gray to black. Pure hematite does not attract a magnet, but sources differ	MSA IC MSA IIA WUSTL ASU
Garnet (almandine)	7 to 7½	Only scratches the plate*	Vitreous	3.85 to 4.20	Brown to red, violet-red or orange-red; almandine is the most common garnet	MSA IIIB-2
Mica (muscovite)	2 to 2½	White or none	Pearly on flat surfaces	2.76 to 2.88	White, pale green or pale yellow; thin sheets and flakes bend back; weathers golden	MSA IIB-1
Basalt	—	—	Fine-grained or glassy texture	—	Lava rock; trapped gas can leave a bubbly texture	USGS
Granite	—	—	Coarse-grained texture	—	Cooled slowly underground, so its crystals are big enough to see	USGS
Sandstone	—	—	Sand-sized grains	—	Often layered; most of the Southwest’s red rocks are sandstone cemented by hematite	USGS WUSTL
Limestone	3*	—	—	—	Made of calcite, so it should fizz in vinegar*; often layered	USGS USGS MSA IIC-1

* Our inference from the sourced values. A dash means we found no source for that value. Rocks are mixtures of minerals, so they have no single hardness, streak or specific gravity. MSA is the Mineralogical Society of America’s identification key; WUSTL is Washington University in St. Louis.

Why this key does not use photos

The USGS says rocks and minerals must be examined in person from all sides for accurate identification, and are extremely difficult to identify from photographs. Simple tests on the rock in your hand tell you things a photo cannot: how hard it is, what color its powder is, whether a magnet sticks. If you are still unsure, the USGS suggests a state geological survey, a natural science museum, a university geology department, a rock shop or a gem and mineral club.

Keep a log of your finds

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

Questions

Which common minerals will scratch glass?

Anything harder than window glass, about 5½: quartz, chalcedony, agate and jasper (7), garnet (7 to 7½), feldspars and pyrite (6 to 6½) and magnetite (6), by the Mineralogical Society of America’s values. Washington University in St. Louis notes that quartz is the only common mineral that scratches glass easily.

Which minerals are magnetic?

Magnetite is strongly magnetic. Pure hematite does not attract a magnet, according to Washington University in St. Louis, though Arizona State University’s outreach page lists it as magnetic. Slag, an industrial look-alike, is often magnetic.

Which rocks fizz in vinegar?

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

What color streak do common minerals leave?

Hematite rust red; magnetite black; pyrite black to greenish black; calcite white; turquoise white to pale green. Minerals harder than the streak plate, about 6½, only scratch it.

Is Oregon sunstone a feldspar?

Yes. GIA describes it as natural copper-bearing labradorite, a plagioclase feldspar, and Oregon’s geology agency calls it a feldspar crystal that weathers out of lava flows.

How can I tell opal from quartz?

Opal is softer, 5 to 6 against 7, and lighter, with a specific gravity of 1.9 to 2.1 against 2.65.

Can an app identify my rock from a photo?

The USGS says rocks are extremely difficult to identify from photographs and should be examined in person. Treat any photo result as a starting point and check it with simple tests like the ones in this key.

Sources

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