Forsterite
Forsterite is the magnesium-rich end member of the olivine mineral series, with the formula Mg2SiO4, and its gem-quality transparent green variety is known as peridot. It crystallizes directly from magnesium-rich magma and is one of the first minerals to form as basaltic and ultramafic melts cool. Forsterite is abundant in Earth's upper mantle, where the olivine-rich rock peridotite is believed to make up much of that layer.
Forsterite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Mg2SiO4 |
| Mohs hardness | 6.5–7 |
| Crystal system | Orthorhombic |
| Luster | Vitreous |
| Streak | White |
| Cleavage | Poor |
| Fracture | Conchoidal |
| Specific gravity | 3.2–3.3 |
| Transparency | Transparent to translucent |
| Common colors | Pale yellow-green, Olive green, Colorless |
| Rarity | common |
Geology & background
Forsterite crystallizes at high temperature from mafic and ultramafic magmas, appearing as an early-formed mineral in basalt, gabbro, and peridotite, and it also occurs in metamorphosed impure dolomitic marbles where magnesium-rich carbonate reacts with silica during metamorphism. Because it forms deep within the mantle, forsterite-rich olivine is frequently carried to the surface as xenocrysts within volcanic rocks, particularly kimberlite pipes, and within olivine nodules scattered across basalt flows in places like Arizona's San Carlos Apache Reservation and the Hawaiian Islands.
Collectors most often encounter forsterite as gem peridot, whose vivid yellow-green color comes from iron substituting for magnesium in the crystal structure, or as loose rounded olivine grains weathered out of basalt. Peridot is unusual among gems in never requiring heat treatment, since its color is stable and intrinsic to the crystal. Rockhounds should note that olivine weathers relatively quickly at Earth's surface, altering to serpentine minerals, so fresh, unweathered material collects best.
How to identify Forsterite
Forsterite-olivine typically forms as rounded, granular grains or short stubby crystals with a glassy, vitreous luster and colors ranging from pale yellow-green to deep olive green, with a white streak. Hardness is 6.5 to 7, and it has poor cleavage but a conchoidal fracture typical of many silicates. Transparent gem-quality material is called peridot. It is most often confused with green tourmaline or demantoid garnet, but olivine's lower birefringence, characteristic doubling of facet edges under magnification, and typical yellow-green hue help separate it from those look-alikes.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Forsterite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Forsterite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Peridot, the gem variety of forsterite, is one of the few gemstones that forms in essentially a single color rather than a range.
- Olivine-rich peridotite is thought to make up much of Earth's upper mantle, tying forsterite directly to the planet's deep interior.
- Loose peridot crystals occasionally arrive at the surface embedded in meteorites, specifically pallasites, alongside metallic iron-nickel.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
.jpg?width=320)
.jpg?width=320)