Hypertext Webster Gateway: "obsidian"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
Obsidian \Ob*sid"i*an\, n. [L. Obsidianus lapis, so named,
according to Pliny, after one Obsidius, who discovered it in
Ethiopia: cf.F. obsidiane, obsidienne. The later editions of
Pliny read Obsianus lapis, and Obsius, instead of Obsidianus
lapis, and Obsidius.] (Min.)
A kind of glass produced by volcanoes. It is usually of a
black color, and opaque, except in thin splinters.
Note: In a thin section it often exhibits a fluidal
structure, marked by the arrangement of microlites in
the lines of the flow of the molten mass.
From WordNet (r) 1.7 (wn)
obsidian
n : acid or granitic glass; usually dark, but transparent in
thin pieces
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