Hypertext Webster Gateway: "tridymite"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
Tridymite \Trid"y*mite\, n. [Gr. ? threefold. So called in
allusion to the supposed threefold nature of the twin
crystals.] (Min.)
Pure silica, like quartz, but crystallizing in hexagonal
tables. It is found in trachyte and similar rocks.
From WordNet (r) 1.7 (wn)
tridymite
n : a mineral form of silica
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