Fluorite
CaF₂ · Cubic · also called fluorspar
Fluorite is the colour showpiece of common minerals and the hardness-4 reference on the Mohs scale. Purple fluorite is regularly mistaken for amethyst, but amethyst is hardness 7 with no cleavage — a steel nail settles it immediately.
Properties
- Mohs hardness
- 4
- Streak
- white
- Lustre
- vitreous
- Colour
- Famous for colour zoning — a single crystal can be purple at the corners and green in the middle. Purple and green are the most common.
- Cleavage
- Perfect octahedral cleavage in four directions. Damaged corners break away as little triangular pyramids, which is very distinctive.
- Fracture
- Conchoidal
- Specific gravity
- 3–3.25 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Some specimens fluoresce
Tests that confirm fluorite
- Hardness exactly 4 — a steel nail scratches it, a copper coin does not.
- Many specimens glow blue or violet under ultraviolet light. The word 'fluorescence' was named after this mineral.
- Cubic crystals with cleavage cutting the corners off diagonally.
What fluorite gets confused with
Each of these is settled by one test. Run that test first.
Amethyst is hardness 7 with no cleavage. Fluorite is scratched by a nail and shows flat cleavage faces.
Calcite fizzes in acid and is softer; fluorite does not react.
Where it is found
Hydrothermal veins, often with galena, sphalerite, barite, and quartz. Classic sources in Illinois, Cumbria, and southern China.
Is it worth anything?
Common material is inexpensive; well-formed, colour-zoned crystal groups from named localities can be genuinely valuable. Too soft and too cleavable for jewellery use.