Olivine is a magnesium-iron silicate and one of the most abundant minerals in the Earth's upper mantle. At the surface it appears in basalt as distinctive yellow-green crystals or masses, and gem-quality material is the birthstone peridot. It weathers readily — iron oxidation turns surface olivine rusty brown (to iddingsite), so fresh green color indicates unweathered material. The characteristic olive-green to yellow-green color combined with a glassy luster and occurrence in dark basaltic rocks is usually enough for field identification.
Olivine (Peridot) Identification Sheet
The green mineral that makes up much of the Earth's mantle — and surfaces as gemstone peridot when conditions align.
Luster
Vitreous
CleavagePoor; conchoidal to uneven fracture
Common lookalikes
Epidote (yellow-green but more yellow, different crystal form); green tourmaline (striated, harder); demantoid garnet (more brilliant luster, dodecahedral). Tip: find it in basalt — olivine is almost always the only green thing in a dark volcanic rock.