In colour they vary from ash-grey to black, and their fracture is conchoidal.
The cleavage is imperfect, but there is a well-marked conchoidal fracture.
Occasionally the rounded cracks extend from the matrix into some of the crystals especially those of quartz which have naturally a conchoidal fracture.
The chemical composition of typical obsidians is shown by the following analyses Obsidian, when broken, shows a conchoidal fracture, like that of glass, and yields sharp-edged fragments, which have been used in many localities as arrow-points, spear-heads, knives and razors.
They pass through a viscous stage in cooling from a state of fluidity; they develop effects of colour when the glass mixtures are fused with certain metallic oxides; they are, when cold, bad conductors both of electricity and heat, they are easily fractured by a blow or shock and show a conchoidal fracture; they are but slightly affected by ordinary solvents, but are readily attacked by hydrofluoric acid.