Obsidian
Reference articles are in English.
A specimen of obsidian, the natural volcanic glass. Source
Description
Obsidian is natural volcanic glass, commonly black or dark brown. Bands, pale inclusions and optical effects occur in some varieties. It fractures into curved surfaces that can form very sharp edges. The treatment of projections and edges is therefore important in carved or polished objects.
History
Obsidian forms when molten rock cools without developing a normal coarse crystalline texture. Its glassy structure results from that cooling history, rather than from an applied surface treatment.
Commissioning
The maker and client should agree how the finished object will be displayed, handled and supported. A sample can establish the proposed surface. An object intended for daily contact requires particular attention to exposed edges and vulnerable details.