1. A eutectic material formed between ferrite and cementite. 2. A type of obsidian found in California that is 70-75% silica, 12-14% alumina and 6-8% alkalis. Perlite contains a high proportions of cracks, spheroids and water. When it is flash roasted to 10008C, perlite expands to 20 times its size forming bubble-filled beads. This white aggregate is then crushed and used as an extender. Expanded perlite is also used to make insulating bricks. Synonyms: Pearlite. Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]