Geologists identify a mineral with a few simple tests. Each test measures something set by the mineral's chemistry and crystal structure.
- Hardness: what it can scratch. The Mohs scale ranks ten minerals from talc (1) to diamond (10). A fingernail is about 2.5, a copper coin 3.5, a steel nail or glass 5.5.
- Streak: the color of its powder, made by rubbing it on unglazed white tile.
- Luster: how its surface reflects light: metallic or nonmetallic (glassy, pearly, earthy).
- Cleavage or fracture: whether it splits along flat planes (cleavage) or breaks along rough or curved surfaces (fracture).
- Density: galena (7.5 g/cm³) feels far heavier than quartz (2.65 g/cm³) of the same size.
- Special tests: magnetite attracts a magnet; calcite fizzes in weak acid.
Key idea
Identify minerals with several tests together: hardness, streak, luster, cleavage and density. Color alone can fool you.
Why it works
Color often comes from tiny traces of other elements. Pure quartz is clear, but a little iron makes it purple (amethyst). Streak is more reliable, because powdering the mineral spreads out those traces: hematite can be steel gray or red-brown in a chunk, yet its streak is always red-brown. Hardness depends on how strongly the atoms are bonded, which is the same in every sample.
Worked example: Narrowing hardness
A mineral scratches glass (5.5) but cannot scratch quartz (7). Quartz scratches it. What can you say about its hardness?
- It scratches glass, so it is harder than 5.5.
- Quartz scratches it, so it is softer than 7.
- Its hardness is between 5.5 and 7, so it might be orthoclase feldspar (6).
Answer: Between 5.5 and 7
Worked example: Same color, different minerals
Two shiny gold-colored samples: A leaves a greenish-black streak and B leaves a golden-yellow streak and bends like soft metal. Which is gold?
- Gold's streak is golden yellow, and it is soft (2.5 to 3).
- A greenish-black streak and a brassy color mark pyrite, "fool's gold".
Answer: B is gold; A is pyrite
Common mistake
Identifying minerals by color, or reading Mohs 10 as ten times harder than 1. One mineral can come in many colors, so use streak and other tests. And the Mohs scale is only a ranking, like places in a race: diamond (10) is far more than ten times as hard as talc, and the steps are not equal. Check: say "harder than" or "softer than," not "twice as hard."