Identification guide · Minerals
How to Identify Biotite
Biotite appears black to dark brown or greenish-black, with a distinctive vitreous to pearly luster on its cleavage faces. Its hardness is low, around 2.5 to 3, easily scratched with a fingernail, and its streak is white to pale gray. The defining test is its perfect basal cleavage, which allows thin sheets to peel off in flexible, elastic layers that spring back after bending, unlike brittle chlorite, which it superficially resembles. Biotite is often mistaken for muscovite, but muscovite stays colorless to pale, while biotite is consistently dark.
Biotite identification checklist
| Color | Black, Dark brown, Dark green | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous to pearly (splendent on cleavage) | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Translucent in thin sheets, opaque in mass | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 2.5–3 | A copper coin (Mohs 3.5) scratches it, but a fingernail does not. |
| Streak | White to gray | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect basal {001}, one direction (micaceous) | |
| Fracture | None; flexible, inelastic cleavage flakes | |
| Specific gravity | 2.7–3.4 | Ordinary rock-like heft (specific gravity about 3.05), similar to quartz. |
| Crystal system | Monoclinic |
Biotite look-alikes and how to tell them apart
Both sit near Mohs 2.5–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Biotite vs Muscovite →
Both sit near Mohs 2.5–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Biotite vs Calcite →
Both sit near Mohs 2.5–3, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Biotite vs Phlogopite →
Aegirine is harder (Mohs 6–6.5) than Biotite (Mohs 2.5–3), so Aegirine will scratch Biotite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
Biotite identification FAQ
How can you tell if a rock is Biotite?
Check the color (black, dark brown, and dark green), the vitreous to pearly (splendent on cleavage) luster, a Mohs hardness of 2.5–3 and a white to gray streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Biotite?
Biotite has a Mohs hardness of 2.5–3. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
What is the difference between Biotite and Muscovite?
Biotite (Mohs 2.5–3, vitreous to pearly (splendent on cleavage) luster) and Muscovite (Mohs 2–4, vitreous to pearly/silky luster) look alike, but hardness, streak and heft usually separate them. See the full Biotite vs Muscovite comparison for a side-by-side table.
Is Biotite worth anything?
Entry-level Biotite specimens sell for around $2, typical collector pieces about $20, and premium examples $150 or more depending on size, quality and locality.
What color is Biotite?
Biotite is most often black, dark brown, and dark green. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
