Minerals and Identification
The tests that tell one mineral from another.
A mineral is a naturally occurring inorganic solid with a definite chemical composition and an ordered internal crystal structure. Rock is made of minerals; the mineral is the ingredient, the rock is the mixture.
Identification tests
| Property | What it is | Note |
|---|---|---|
| Hardness | Resistance to scratching | Mohs 1–10; the most diagnostic single test |
| Streak | Colour of the powder on unglazed tile | More reliable than the mineral's own colour |
| Lustre | How it reflects light | Metallic, glassy, pearly, dull |
| Cleavage | Tendency to break on flat planes | Reflects the crystal lattice |
| Fracture | Breakage without cleavage | Conchoidal in quartz and glass |
| Density | Mass per volume | Distinguishes look-alikes such as pyrite and gold |
The Mohs scale
Talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. It is ordinal, not proportional — diamond is not ten times harder than talc, it is many times harder than corundum alone. Useful field references: a fingernail is about 2.5, a copper coin 3.5, a steel blade 5.5, glass 5.5.
Colour is the least reliable test
Quartz occurs clear, purple, pink, grey and black depending on trace impurities. Streak and hardness stay constant. Beginners lead with colour and get it wrong; start with hardness and streak.