In reply to:

i think obsidian is formed by volcanos-- certainly, it is almost always black.

Obsidian is black? One of my favourite exhibits at the Museum of Mankind in London (I have this vague idea the museum is not there any more, having been reabsorbed into the British Museum) was a "glass" Aztec skull. I'm sure it was described as being made of obsidian but was the normal transparent non-colour of ordinary glass. It really was beautifully carved and a fascinting object.

Bingley


Bingley