American Dictionary of the English Language

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OBSID'IAN, noun A mineral of two kinds, translucent and transparent. The translucent has a velvet black color; the transparent is of a dark blue. These occur massive in porphyry, gneiss or granite, generally invested with a gray opake crust.

The fracture of obsidian is vitreous or pearly; hence the two varieties, vitreous obsidian and pearlstone.