Obsidian

[əb'sɪdɪən]

(noun.) acid or granitic glass formed by the rapid cooling of lava without crystallization; usually dark, but transparent in thin pieces.

Inputed by Adeline--From WordNet

(n.) A kind of glass produced by volcanoes. It is usually of a black color, and opaque, except in thin splinters.

Typed by Garrett

n. a natural glass—the vitreous condition of an acid lava.

Checker: Polly