1) Volcanic glass 
Tachylite (also spelled tachylyte) is a vitreous form of basaltic volcanic glass. This `glass` is formed naturally by the rapid cooling of molten basalt. It is a basic type of igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster. It is very brittle and oc.....
Tachylite (Tachylyte) is a natural glass, formed by the rapid cooling of molten basalt. It is a black or dark- brown and greasy looking substance. It is very brittle and occurs in basaltic obsidians in dikes, veins and intrusive masses.
[n] - a basic or basalt glass
noun a basic or basalt glass