Sphalerite
(Zn,Fe)S · Cubic · also called zinc blende, black jack
Sphalerite is the main ore of zinc and one of the more confusing common minerals, because its colour ranges from near-transparent yellow to opaque black. The resinous lustre and the six-direction cleavage are constant, and the pale streak from a black-looking specimen is a reliable surprise.
Properties
- Mohs hardness
- 3.5–4
- Streak
- pale yellow, brown
- Lustre
- resinous, adamantine, submetallic
- Colour
- Iron-poor sphalerite is honey yellow or green and gemmy; iron-rich material is near black. The resinous, slightly sticky-looking lustre is consistent across all of them.
- Cleavage
- Perfect cleavage in six directions. Broken faces catch light from many angles at once, which produces its characteristic glitter.
- Fracture
- Conchoidal where cleavage does not control the break
- Specific gravity
- 3.9–4.1 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Some specimens fluoresce
Tests that confirm sphalerite
- Six cleavage directions produce a many-faceted sparkle unlike anything else at this hardness.
- Scratching or striking it can release a faint rotten-egg smell of hydrogen sulfide.
- Streak is pale yellow to light brown even when the specimen looks black.
What sphalerite gets confused with
Each of these is settled by one test. Run that test first.
Garnet is much harder (6.5–7.5) and has no cleavage. Sphalerite is scratched by a copper coin.
Cassiterite is far harder and considerably heavier.
Where it is found
Hydrothermal veins, usually alongside galena, in limestone-hosted lead-zinc districts.
Is it worth anything?
Common massive sphalerite is inexpensive. Transparent gem-grade material is rare, highly dispersive, and genuinely valuable — but too soft for rings.