Cassiterite
SnO₂ · Tetragonal · also called tinstone, wood tin
Cassiterite is the world's only significant ore of tin and the reason Cornwall and the Erzgebirge mattered in European history. It is heavy, hard, and dark, and the density-plus-hardness combination separates it from almost everything else — heavy minerals are usually soft, and hard minerals are usually light.
Properties
- Mohs hardness
- 6–7
- Streak
- white, pale brown
- Lustre
- adamantine, submetallic, greasy
- Colour
- Usually near black or dark reddish brown. Thin edges are often translucent red-brown when held to a strong light, which is a useful hint.
- Cleavage
- Poor cleavage; it mostly fractures.
- Fracture
- Subconchoidal to uneven
- Specific gravity
- 6.8–7.1 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm cassiterite
- Extraordinarily heavy — nearly as dense as galena, but hard rather than soft. That combination is close to unique.
- Scratches glass easily, unlike most other very heavy minerals.
- Pale brown or white streak despite the near-black appearance.
What cassiterite gets confused with
Each of these is settled by one test. Run that test first.
Sphalerite is much softer — a copper coin scratches it — and shows six cleavage directions.
Rutile is lighter and more often a clear red-brown, with slender striated prisms.
Magnetite is strongly magnetic with a black streak; cassiterite is neither.
Where it is found
Granite pegmatites, high-temperature hydrothermal veins, and the placer deposits downstream of them, where its weight and durability concentrate it in stream gravel.
Is it worth anything?
Ordinary material has modest value. Sharp twinned crystals from classic localities are genuinely collectible.