Obsidian

Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.

Colour: Shiny black, also brown or grey.

Grain size: None; the rock is glassy.

Texture: Glassy, but obsidian may contain numerous phenocrysts.

Structure: May be spotted or flow banded and spherulites (see rhyolite) are common. Being a siliceous glass it breaks with a conchoidal fracture and may be fashioned to a sharp cutting edge. It was used for cutting tools by primitive peoples.

Mineralogy: Essentially a glass. Rare phenocrysts of quartz and feldspar.

Field relations: Dykes and flows. Commonly associated with rhyolites to which they are chemically equivalent.