Emily Dickinson Lexicon

OB-SID'I-AN, n.

Glossy lava. 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. Dict. Nat. Hist. Ure. Kirwan. The fracture of obsidian is vitreous or pearly; hence the two varieties, vitreous obsidian and pearlstone. Jameson.