Tachylite

Tachylite, a black opaque natural glass, which results from the rapid cooling of molten basalt. It occurs as a thin selvage to dykes and veins of intrusive basalt—the latter having cooled rapidly from contact with the adjacent rocks. In Hawaii it appears as a scoriaceous or a compact crust, 2 inches or less in thickness, upon the basic lavas of that region. Tachylite never forms lava-flows like the acidic glasses (Obsidian). See BASALT.