Construction Materials: Lightweight Aggregates. A.L. Bush, in Encyclopedia of Materials: Science and Technology, 2001 (a) Perlite Perlite, obsidian, and pitchstone are volcanic glasses that differ in water content: obsidian, less than 2% H 2 O; perlite, 2–5%; and pitchstone, more than 5%. The general range of composition (Bush 1973, p. 341) is …