perlite

noun

1. An amorphous volcanic glass formed by the hydration of obsidian.
2. The lightweight insulating material and aggregate resulting from expanding perlite glass by heat.

   “Technical terms like ferrite, perlite, graphite, and hardenite were bandied to and fro, and when Paget glibly brought out such a rare exotic as ferro-molybdenum, Benson forgot that he was a master ship-builder, […]”