Obsidian is often referred to as volcanic glass, that has cooled too quickly for crystals to form. It is a silica-rich natural glass and typically is very black, shiny, glassy, with conchoidal fracture (smooth curved breaks, often with concentric ridges). Obsidian occurs as crusts on lavas, as complete flows or as near surface volcanic domes. Obsidian may be red-brown due to the presence of iron. Its conchoidal fracture can produce very sharp edges and its use for knives and arrows makes it an important archaeological material.
Colour: black, dark colours.
Mineralogy: essential glass, other minerals can include alkali feldspar, quartz.
Classification: intermediate/ acid igneous rock.
Occurrence: not widely distributed although locally common.
Texture: none; massive glass with conchodial fracture.
Structure: glass may contain spherical bodies (spherulites), may be spotted or show flow banding.
Obsidian is often referred to as volcanic glass, that has cooled too quickly for crystals to form. It is a silica-rich natural glass and typically is very black, shiny, glassy, with conchoidal fracture (smooth curved breaks, often with concentric ridges). Obsidian occurs as crusts on lavas, as complete flows or as near surface volcanic domes. Obsidian may be red-brown due to the presence of iron. Its conchoidal fracture can produce very sharp edges and its use for knives and arrows makes it an important archaeological material.