OBSIDIAN, a glassy volcanic rock of acid composition.

On this account the fibres of tussur or tussore silk tend to split up into fine fibrillae under the various preparatory processes in manufacturing, and its riband structure is the cause of the glassy lustre peculiar to the woven and finished fibres.

Wings of both pairs closely alike; firm and glassy in texture.

The minute globular bodies have occasionally a sub-pearly lustre, and glassy rocks which possess this structure have been called perlites (q.v.).

Included in this group are some rocks which are more properly to be regarded as vitreous forms of trachyte than as glassy rhyolites (Iceland), but except by chemical analyses they cannot be separated.