Volcanic glass - a rock rather than a mineral- is a mixture of cryptocrystalline grains of silica minerals in a glass-like suspension, a super-cooled liquid. Obsidian is formed in the latest stage of volcanic eruptions, the silica left over after most of the other elements and water has been used up are ejected or flow out and rapidly chilled at surface temperatures.
Colour:black, bluish, mahogany, golden, peacock, etc. - the colors are due mainly to refraction by microscopic bubbles (and microscopic mineral inclusions such as magnetite in "Rainbow Obsidian").
Glassy, fresh igneous rocks with a high silica content and conchoidal fracture. The earliest manufactured mirrors were pieces of polished obsidian.