Igneous Rocks - Obsidian

"Obsidian and pitchstone
Colour: Shiny black, also brown or grey. Pitchstones have a dull rather than a shiny lustre.

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 (abundant in pitchstones) of quartz and feldspar.

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

(Hamilton et al 1976, 170)