What is black granite made of?

[allen]

I know it isn't granite, per se, so what is it, and how is the black polished surface achieved?

Ta.


A

[ChuckFickens]

Generally...

http://en.wikipedia.org/wiki/Basalt

[The Builder]

Polished in much the same way that concrete worktops are. These are a big deal in the USA, particularly in California right now...........