Olivine Group (Forsterite and Fayalite) - (Mg,Fe)₂SiO₄

Habit: Pale yellow to olive green, greenish yellow, yellow brown or brown to black; color intensifies with increasing iron content. Subhedral to euhedral crystals, typically thick with striations, with wedge-shaped terminations; commonly granular or compact massive. Vitreous to resinous luster on fractures, gradating to dull or submetallic; transparent to translucent. White streak.

Environment: Occurs in ultramafic volcanic and plutonic rocks; less common in felsic plutonic rocks.

Etymology: Forsterite was named after Adolarius Jacob Forster (1739-1806), a noted English mineral collector. Fayalite was named for the locality where it was first discovered, Fayal (or Faial) Island in the Azores.