Glossary
Mohs Hardness Scale
A relative scale from 1 (softest) to 10 (hardest) used to describe scratch resistance.
| Hardness | Reference mineral |
|---|---|
| 1 | Talc |
| 2 | Gypsum |
| 3 | Calcite |
| 4 | Fluorite |
| 5 | Apatite |
| 6 | Orthoclase |
| 7 | Quartz |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Diamond |
Luster
How a mineral’s surface reflects light.
- Vitreous
- Glassy (e.g. quartz)
- Metallic
- Reflective like metal (e.g. pyrite)
- Adamantine
- Brilliant, diamond-like
- Silky
- Fine fibrous sheen
- Waxy
- Dull, waxy appearance
- Earthy
- Dull, no reflectance
Crystal Systems
The seven crystal systems describe the symmetry of a mineral’s unit cell.
| System | Axes | Example minerals |
|---|---|---|
| Cubic | a = b = c, all 90° | Pyrite, halite |
| Tetragonal | a = b ≠ c, all 90° | Zircon, rutile |
| Orthorhombic | a ≠ b ≠ c, all 90° | Olivine, topaz |
| Hexagonal | a = b ≠ c, 120° & 90° | Beryl, apatite |
| Trigonal | a = b ≠ c, 120° & 90° | Quartz, calcite |
| Monoclinic | a ≠ b ≠ c, two 90° one ≠ | Gypsum, orthoclase |
| Triclinic | a ≠ b ≠ c, no 90° | Plagioclase, kyanite |
Habit
The characteristic external shape or form of a crystal.
- Prismatic
- Elongated, column-like crystals
- Tabular
- Flat, plate-like crystals
- Botryoidal
- Rounded, grape-like masses (e.g. malachite)
- Massive
- No visible crystal faces
- Acicular
- Needle-like crystals
- Druzy
- Coating of small crystals on a surface