Olivine (Peridot)
(Mg,Fe)₂SiO₄ · Orthorhombic · also called peridot, chrysolite
Olivine is the main mineral of the Earth's upper mantle, and it arrives at the surface as green nodules carried up in basalt. Gem-quality olivine is peridot. The green sand beaches of Hawaii are made of it.
Properties
- Mohs hardness
- 6.5–7
- Streak
- white, colorless
- Lustre
- vitreous
- Colour
- A distinctive olive to bottle green, sometimes verging on yellow.
- Cleavage
- Poor cleavage; it fractures conchoidally instead.
- Fracture
- Conchoidal
- Specific gravity
- 3.27–3.37 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm olivine
- Sugary green grains packed together in dark volcanic rock — a very recognisable texture.
- Hardness near 7, so it scratches glass.
- Often found as rounded nodules of green crystals inside basalt.
What olivine gets confused with
Each of these is settled by one test. Run that test first.
Epidote is a yellower pistachio green and forms elongated crystals with one good cleavage.
Serpentine is much softer and waxy; olivine is hard and glassy.
Where it is found
Basalt, gabbro, and peridotite; as nodules in volcanic rock and as green sand on some volcanic beaches.
Is it worth anything?
Peridot is a moderately priced gemstone. Ordinary olivine grains have no value.