This showed up in an old crystal collection and we were intrigued. It wasn't the reason we bought the collection, nor is it a mineral we hae ever bought for ourselves. I am not sure if you can really call it a mineral specimen because it is man made? At any rate, they make this stuff as a harsh abrasive for industrial purposes. The thing is that they make it and it is really, reallly pretty.
 
Carborundum can express sort of rainbow colours. This piece trends towards black and blue shades. It is pretty dark with lots and lots of reflective faces in there. It is Scotland in winter so I don't have the opportunity to see this in the bright sunlight but I bet its gorgeous.
 
This chunk of carborundum is notoriously hard - usually it's around 9 on the Mohs scale (quartz is 7 by comparison)- but this piece is brittel, too, so little bits are flaking off with handling. Our advice would be to put it on a shelf and not handle it. This mineral is to be looked at and admired rather than touched.
 
Interesting FYI: silica carbide grit which we use for rock tumbling comes from this abrasive.
 
RRP large piece is £30. This piece is medium to large and second hand.