Obsidian

["Susan" <zdome@xxxxxxxx>]

Obsidian, in the form called 'Apache Tears' are polished in tumblers and
often found around Arizona as tourist trinkets.  I have spent some hours
mining these locally and generally a polished small stone would sell for
25cents or so.  In raw form they are small round black lumps embeded within
the softer pale grey rock around.   I would imagine with a large enough
piece sand blasting could produce some interesting results, also perhaps
chemical etching.  But for carving, I don't see how, except perhaps with
very high speed diamond drills (like a dentist drill?) that might do it
without shattering the stone.  This material is literally 'natural glass'
and does not hold the characteristics of a good carving rock at all.
Beautiful stuff though :).

Historically, the flat surfaces of the larger pieces found further south,
were used in ancient times as mirrors since the conchoidal broken surface
could provide areas with good reflection.