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Black Rocks and Minerals: How to Tell 12 Look-Alikes Apart

October 1, 2026 / 9 min read / RockhoundID Team

Black rocks turn up everywhere: on beaches, in creek beds, in driveway gravel and along railroad tracks. Some are ordinary volcanic rock, some are interesting minerals and a few are leftovers from a furnace. From a few steps away they all look the same, which is exactly why color is no help here.

What two tests identify most black rocks?

Test 1: the streak

Rub a corner of the rock firmly across the unglazed back of a white bathroom tile and look at the powder. The Washington University in St. Louis meteorite site says hematite and magnetite streaks are easy to make, "almost like chalk on a sidewalk": hematite's is rust or blood red, magnetite's dark gray. Most other black minerals leave a white, pale brown or green streak. Anything harder than about 6.5 leaves no streak at all, because it is harder than the tile. Our streak test guide covers the method in detail.

Test 2: the magnet

Use a cheap fridge magnet, not a strong rare-earth one, which pulls on many ordinary rocks. A handy trick from the same site: stand the magnet on its edge on a table and bring the rock close. If the rock attracts it, you can make the magnet roll by pulling it along with the rock. Magnetite attracts it strongly. Pure hematite does not.

After those two, a steel nail or knife (about 5 to 5.5 on the Mohs scale), a piece of glass (about 5.5) and a hand lens break the remaining ties. Our guide to magnetic rocks goes further into what makes a rock pull a magnet.

Black rock look-alikes at a glance

"Skip" means the test is not useful for that rock, usually because it is a mixture of minerals or too varied to give one answer.

What are the 12 black look-alikes?

1. Obsidian

Obsidian is natural volcanic glass, usually rhyolite in composition, that formed when lava cooled too fast for crystals to grow. Oregon State University's Volcano World gives it a typical hardness of 5 to 5.5 and says its jet-black color comes from microscopic crystals of minerals such as magnetite, hornblende and pyroxene. It breaks with smooth, curved (conchoidal) surfaces whose edges can be sharper than a razor. More in our obsidian guide.

2. Basalt

Basalt is the everyday black rock. The University of Auckland calls it the most widespread of all igneous rocks, more than 90 percent of all volcanic rocks, dark gray to black and fine grained. Look with a lens: you will see tiny grains, sometimes green olivine specks, sometimes round gas holes (vesicles). Dull and grainy rather than glassy is the tell.

3. Magnetite

Magnetite is black, metallic and heavy, about 5.2 times as heavy as water against about 2.7 for an average rock. It streaks black and is a natural magnet, which is how it got its name. It forms eight-sided crystals, grains in many igneous rocks and black sand on some beaches.

4. Hematite

Hematite can be steel gray, silvery or black, often in lumpy, kidney-shaped masses, and it is about 5 to 6 on the Mohs scale. Whatever the outside looks like, the streak is cherry red to reddish brown. That single test separates it from magnetite, and pure hematite does not attract a magnet.

5. Black tourmaline (schorl)

Schorl forms long prisms with fine grooves running along their length and a rounded, triangular outline when seen end on. It is 7 to 7.5 on the Mohs scale, so it scratches glass, and it is common in granites and pegmatites (very coarse-grained granite). Its streak is white, though at that hardness it barely marks a tile.

6. Hornblende

Hornblende gives many granites and diorites their dark specks. It is dark green to black, about 5 to 6 in hardness, and leaves a dark green streak. With a lens, its two cleavages (flat breaking directions) meet at about 56 and 124 degrees, like a squashed diamond.

7. Augite

Augite looks almost identical to hornblende and lives in basalt and gabbro. The difference is geometry: its two cleavages meet at close to right angles, and its stubby crystals have square-ish cross sections. Its streak is greenish white, paler than hornblende's.

8. Biotite

Biotite is black mica, brown to black and soft (2.5 to 3). Its one perfect cleavage lets it peel into thin, shiny sheets with a fingernail or a pin. You will see it as glittering black flakes in granite, schist and gneiss. Its streak is very pale brown.

9. Coal

The USGS describes coal as a black or brownish-black sedimentary rock made mostly of carbon, and readily combustible. Bituminous coal is blocky and shiny, with thin alternating shiny and dull layers; anthracite is hard, brittle and lustrous. Coal often feels light for its size and turns up in layers among other sedimentary rocks. Do not try to burn a sample to test it.

10. Jet

Jet is a compact, velvet-black coal that takes a good polish, which is why it has been carved into beads and jewelry. The USGS lists it at 2.5 to 4 on the Mohs scale with a specific gravity of 1.30 to 1.32, only about half as heavy as quartz. Very light and soft is what separates it from obsidian and onyx.

11. Black onyx

Onyx is a banded form of chalcedony, the microcrystalline quartz family that includes agate and jasper. GIA notes that what many call "black onyx" is actually dyed black chalcedony, because natural black chalcedony is rare. It is about 7 on the Mohs scale, scratches glass and is smooth, waxy and evenly black. You are far more likely to meet it as beads than in a creek bed.

12. Slag

Slag is the glassy, bubbly waste from smelting and furnaces, and it is common in fill, gravel and near old industry. The Washington University in St. Louis meteorite site says a rock is probably slag if it attracts a magnet or has flow features on one surface, because slag often contains metallic iron. Glassy plus gas bubbles means slag. See meteorite or meteorwrong for the full lineup of fakes.

Could a black rock be a meteorite?

It is possible, but the odds are long, and the checks are strict. A meteorite should have a fusion crust, a thin skin rarely more than 1 to 2 mm thick that is almost always darker than the inside. About 95 percent of meteorites attract a cheap magnet, because they contain iron-nickel metal, and ordinary chondrites (most stony meteorites) show shiny metal specks on a sawn or broken face. Very few have gas bubbles.

A black rock that attracts a magnet but shows no metal inside is not a meteorite; magnetite or slag is far more likely. A find from a beach, a stream bed, a roadside or a railroad is almost certainly not one either. Do not clean or test a possible meteorite with a wire brush, abrasives, acid or a torch.

Other black rocks worth a second look

A few more dark stones show up often enough to mention. Scoria is bubbly black or red lava rock, denser than pumice. Black chert and flint break into sharp, curved flakes and scratch glass. Smoky quartz can be so dark it looks black, but it is still glassy quartz that scratches glass and streaks white. If yours looks like glass but is none of the above, try our guide to rocks that look like glass, and for every other color, start from the rock identification by color hub.

The takeaway

Black rocks look alike, but they do not behave alike. A streak on a tile and a fridge magnet split the metallic minerals, a glass scratch and a hand lens sort the crystals, and the heft test picks out coal and jet. The RockhoundID app, coming soon, will suggest likely matches from a photo and tell you which of these tests confirms them. Until then, keep a white tile and a magnet in your pocket, and most black rocks will tell you their names.

RockhoundID guides help you narrow down what you found. They suggest likely candidates, not certain identifications or appraisals. For anything valuable, rare or possibly from space, ask a museum, a university geology department or your state geological survey, and check the collecting rules for the land before you take anything home.