Rock and Mineral Identification Reference

How to Identify Biotite (Black Mica): Hardness, Streak, Luster and Tests

Quick answer

Biotite (Black Mica) (mineral): 6 identifying properties, and how to tell it from Muscovite, quoted from geology department and survey sources.

Updated 2026-09-10 · Sources: University of Minnesota Common Minerals

Scratch, streak and acid tests mark the specimen, and some produce sharp fragments or dust. Wear eye protection, work in a ventilated space, keep acid off skin, and never taste a mineral you have not already identified.

How to Identify Biotite (Black Mica): Hardness, Streak, Luster and Tests — Photo: Robert M. Lavinsky · CC BY-SA 3.0 · source

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What are the key facts?

Type: Mineral
Mohs hardness: 2.5–3 (soft)
Streak: White to gray
Luster: Pearly to submetallic luster.
Color: Brown to black (biotite); golden yellow (phlogopite).
Cleavage or fracture: One direction of perfect cleavage, splitting into thin flexible sheets.
Specific gravity: 2.8–3.4 (phlogopite 2.7)

What Biotite (Black Mica) looks like

The properties that identify Biotite (Black Mica) — mohs hardness: 2.5–3 (soft); streak: White to gray; luster: Pearly to submetallic luster; color: Brown to black (biotite); golden yellow (phlogopite); cleavage or fracture: One direction of perfect cleavage, splitting into thin flexible sheets; specific gravity: 2.8–3.4 (phlogopite 2.7). Hardness, streak and cleavage are the reliable ones; color alone rarely settles it.

Often confused with

[Muscovite (White Mica)](/lookup/rocks-and-minerals/muscovite): Biotite is dark-colored (brown to black); muscovite is colorless to silvery-white.

Where it is found in North America

An important constituent of granite and metamorphic rocks in north-central Minnesota and Wisconsin.

How to photograph it for identification

Photograph a fresh, unweathered surface in daylight, then a second shot of any crystal faces or broken edge. Include a coin for scale. If you did a streak test, photograph the streak next to the specimen. The app suggests the likely mineral or rock; confirm with hardness and the tests above before you label it.

FAQ

How hard is Biotite (Black Mica) on the Mohs scale?

2.5–3 (soft). A fingernail is about 2.5, a copper coin about 3, a steel knife 5.5 and glass about 5.5, so test against those.

What color streak does Biotite (Black Mica) leave?

White to gray. Rub the specimen on an unglazed porcelain tile; the powder color is the streak, and it is more consistent than the specimen color.

How do I tell Biotite (Black Mica) from Muscovite?

Biotite is dark-colored (brown to black); muscovite is colorless to silvery-white.

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Sources

University of Minnesota Common Mineralscommonminerals.esci.umn.edu/minerals-f/biotite
University of Minnesota Common Mineralscommonminerals.esci.umn.edu/minerals-g-m/muscovite

Compiled by the RockLens editorial team

Every fact on this page is taken from the sources listed above; numbers and prices are copied from the source, and prices name the guide or auction and its date. Research and drafting were assisted by AI, and each claim was then checked against the cited sources. Checked 2026-09-10. We do not buy, sell or grade items; the app identifies what you photograph and points you to these references.