A glass formed mainly by the hydration of obsidian that has the property of expanding when heated. It softens at 850–900 °C, and water held in its structure vaporizes causing the perlite to expand to seven to sixteen times its original volume. Expansion reduces its bulk density from about 1100 kg/m3 to 30–150 kg/m3. Expanded perlite has many uses in construction and horticulture.