Black rock identification

By The Gleamite desk, Identification research · updated 2026-09-16

Black rocks look alike and test apart. Basalt, the most common lava rock, is the first candidate; beyond it, a magnet flags magnetite, slag and most meteorites, and a streak plate names the heavy ones — magnetite draws black, hematite red-brown, meteorites almost nothing. Glassy black is obsidian or slag, and light black that marks your hands is coal.

Two checks sort black rocks

Black is the color where looks fail completely. Basalt, magnetite, slag and a meteorite can be indistinguishable in a photo, because black hides grain, luster and banding at once. Physical checks do not care about color. A magnet asks whether iron metal or magnetite is present, and a streak plate asks what the powder looks like, and between them they split the usual black finds into small groups. The plate technique itself is on the streak test page.

Stone	Magnet	Streak	Luster and weight	The deciding check
Basalt	None to faint	Gray	Dull, fine-grained, ordinary weight	Grainy matte break, often with small holes
Obsidian	None	White or none	Glassy, light	Curved glassy breaks, no bubbles
Magnetite	Strong	Black	Metallic, very heavy	Magnet plus black streak
Hematite	None or weak	Red-brown	Metallic to dull, very heavy	Red-brown streak
Black tourmaline	None	White or none	Glassy, striated prisms	Rounded triangular cross-section
Black onyx	None	White or none	Waxy, even black, polished	Faint glow at a thin edge against a lamp
Coal	None	Black	Dull to shiny, very light	Marks your hands, light for its size
Flint	None	White or none	Waxy, curved breaks	White rind, sparks against steel
Slag	Often	Varies	Glassy or stony, bubbly, rusty	Round bubbles and rust specks
Meteorite	Usually	None	Dark crust, heavy	Fusion crust, no bubbles, no streak

Run the magnet first, because it is instant and splits the table in two. Then drag anything heavy across unglazed tile. Everything left after those two checks is sorted by luster: glassy, waxy or dull.

The magnetic blacks: magnetite, slag and meteorites

Magnetite is the most strongly magnetic mineral in nature and the commonest heavy black answer to a magnet test: metallic, dense at a specific gravity of 5.2, and it draws a clean black streak. Gold panners know it as the black sand that collects in the bottom of the pan. Hematite looks similar in its silvery black form but leaves a red-brown streak on every specimen, and a hematite that tugs on a magnet is carrying some magnetite with it.

Slag and meteorites both respond to a magnet, so the magnet cannot separate them; bubbles and crust do. Furnace slag is glassy or stony, riddled with round gas bubbles and often flecked with rust, and the full profile of where it comes from and how it fools people is in what is slag glass. A meteorite has no bubbles, no streak and a thin dark fusion crust, and the elimination sequence that retires the impostors before any lab fee is in how to identify a meteorite.

The glassy and waxy blacks

Without a magnet response or a colored streak, luster takes over, and three families remain.

Glassy and conchoidalObsidian: volcanic glass with no crystals and no bubbles, breaking in smooth curved shells with cutting edges. Bubbly glass is slag instead.

Glassy crystalsBlack tourmaline, or schorl: prisms with fine lengthwise grooves and a rounded triangular cross-section, often in granite and pegmatite, hard enough to scratch glass.

Waxy and evenFlint and black onyx: microcrystalline quartz with a waxy luster and curved breaks. Flint carries a pale rind; commercial black onyx is dyed chalcedony.

Obsidian gets the fullest treatment, bubble check and geography rule included, in how to identify obsidian, and flint’s spark and rind checks are in how to identify flint. Schorl, the black tourmaline, is the most commonly found variety of the mineral and grows as an accessory in granite, pegmatite and gneiss, often as scattered black needles that are easy to mistake for flakes of biotite mica until they refuse to peel.

The soft blacks: coal and black shale

Two black rocks fail every hardness test. Coal is organic sedimentary rock, light enough to feel wrong for its size, dull to shiny, and it leaves black marks on your fingers; it also burns, which no other rock on this page does. Black shale is heavier and splits into thin flat sheets along its bedding, and it often smells earthy when breathed on while wet. Both are common and neither is valuable, but both are useful to rule out early, since their softness ends the question in seconds.

Which black rocks are worth something

Value among black finds is concentrated in the rare end. Meteorites are the prize and the most over-claimed. Black opal is a gem with a real market, and its assembled and lab-grown lookalikes are sorted in how to tell if opal is real. Well-formed black tourmaline crystals and jet have collector buyers. Everything else on this page — basalt, obsidian, coal, flint, slag — is worth knowing rather than selling.

Black sits at the edge of the color guides. Dark green serpentine and the olivine grains inside basalt shade in from green rock identification, and the black-veined rhodonite of pink rock identification is the most common way black appears on a stone that is not black at all. For the full family logic of dark igneous, sedimentary and metamorphic rocks, the master tables are in the rock identification chart.

Questions people ask

What type of rock is pure black?

Obsidian is the classic pure black rock, a volcanic glass with a mirror luster and curved, sharp-edged breaks. Coal is dull to shiny black and light for its size. Magnetite is black, metallic and heavy, and black onyx is usually chalcedony dyed to an even black. Basalt runs dark gray to black and looks blackest when wet.

What are some valuable black rocks?

Meteorites are the black finds most likely to carry real value, and also the rarest, since nearly every candidate turns out to be slag or magnetite. Black opal, well-formed black tourmaline crystals and jet have collector markets. Ordinary basalt, obsidian and coal have none beyond curiosity, and black onyx is usually inexpensive dyed chalcedony.

Why is my black rock magnetic?

Magnetite is the usual reason: a black, heavy iron oxide that grabs a magnet and draws a black streak on unglazed tile. Industrial slag often attracts a magnet through trapped metal, and most meteorites respond because they carry iron-nickel. Hematite that pulls on a magnet usually contains some magnetite, and its red-brown streak still gives it away.

Is a heavy black rock a meteorite?

A heavy black rock is rarely a meteorite. Weight and a magnet response fit a meteorite, but they fit magnetite, hematite and slag just as well, and those are far more common. A meteorite also needs a thin dark fusion crust, no gas bubbles and little or no streak on unglazed tile, and only a laboratory confirms one.

The Gleamite deskWrites every identification chart on this site from named mineralogical properties — Mohs values, streak colors, specific gravity — sourced from USGS and standard references on a stated date, and runs the same hardness, streak and vinegar checks it tells readers to run.

Sources

Magnetite is black with a black streak, Mohs 5 to 6.5 and a specific gravity of 5.2, and is the most strongly magnetic common mineral — Geology.com, Magnetite, 2026
Hematite always produces a reddish streak whatever its surface color, has a specific gravity of 5.0 to 5.3, and specimens containing magnetite can attract a magnet — Geology.com, Hematite, 2026
Black is the most common color of tourmaline; it runs 7 to 7.5 on the Mohs scale and grows as striated prisms with rounded triangular cross-sections — Geology.com, Tourmaline, 2026
USGS notes that meteorites lack gas bubbles and should leave no streak on unglazed ceramic, while a black or red streak points to magnetite or hematite — U.S. Geological Survey, meteorite FAQ, 2026
The query black rock identification draws 30 US searches a month at keyword difficulty 0; its parent topic, types of black rocks, carries a traffic potential of 1,200 — Ahrefs Keywords Explorer, US, 2026