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, 164)
black obsidian tool
’banded’ obsidian point
see obsidian point movie