RocksdayBiotite vs Gold
How to tell them apart
Dark, iron-rich mica. The black flecks in granite are often biotite, and weathered flakes in streams can look convincingly golden.
Photo: Fred Kruijen, CC BY-SA 3.0 nl
A native metal that never tarnishes. In nature it occurs as nuggets, flakes in stream gravel and wires in quartz veins.
Photo: Alchemist-hp (talk) www.pse-mendelejew.de, CC BY-SA 3.0 de
Quick tests
- Streak (rub on unglazed porcelain): Biotite leaves a white to gray streak; Gold leaves golden yellow.
- Heft: Gold feels noticeably heavier for its size (specific gravity 15.5–19.3 vs 2.7–3.3).
- Luster: Gold looks metallic; Biotite doesn't.
Key differences
- Weathered biotite glitters gold but floats away when panned and crumbles under a pin.
- Weathered mica flakes float and break apart.
Side by side
| Property | Biotite | Gold |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 2.5–3 | 2.5–3 |
| Luster | Vitreous to submetallic | Metallic |
| Streak | White to gray | Golden yellow |
| Colors | Black, Brown | Gold |
| Crystal system | Monoclinic | Cubic |
| Specific gravity | 2.7–3.3 | 15.5–19.3 |
| Formula | K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂ | Au |
| Found in | Granites and schists worldwide; Bancroft, Ontario | Witwatersrand, South Africa; Carlin Trend, Nevada |
Properties that are the same for both are grayed out.
How to identify biotite
- Black to dark brown flakes that peel into sheets.
- Thin sheets are flexible and elastic.
- Bright, almost metallic shine on flat faces.
- Soft enough to scratch with a fingernail.
Crystal system: Monoclinic
Full Biotite guide →How to identify gold
- Extremely heavy — sinks instantly in a gold pan.
- Bends and flattens instead of breaking.
- Golden streak.
- Stays bright yellow in shade and sun alike.
Crystal system: Cubic
Full Gold guide →More comparisons
Could you tell them apart from a blurry close-up?
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