Cordierite - Mg₂Al₄Si₅O₁₈

Habit: Blue, smoky blue, bluish violet, greenish, yellowish brown or gray. Crystals short prismatic; typically granular to compact or massive. Vitreous luster; transparent to translucent. White streak.

Environment: Occurs in thermally metamorphosed argillaceous sediments and high-grade regionally metamorphosed schists, gneisses and granulites. Also found in mafic igneous rocks and granites and as a detrital mineral.

Etymology: Named in honor of Pierre Louis A. Cordier (1777- 1861), the French mining engineer and geologist, who first studied the species.