[aruanan]

Corundum (also known as carborundum, or emery) is a grinding and polishing material and although very hard (no. 9 on the Mohs hardness scale in which no. 10 is the highest value), it is easy to work in mineral form.

I think rubies and sapphires are also corundum. I have, in some box somewhere, what looks to be a very weathered bipyramidal corundum crystal about the size of a tennis ball I found in a load of gravel in the Dominican Republic (that is, I hope it's there and that someone didn't swipe it from my desk in the lab). Carbide won't scratch it.

[Bernard Marx]

I think rubies and sapphires are also corundum.

They're the gem-quality forms of corundum but most of corundum (aluminum oxide) is sandpaper-quality.

Is your crystal hexagonal? What color? The Dominican Republic seems a very unlikely source for crystalline ruby or sapphire, especially of that huge size. When you say carbide won't scratch it, what form of carbide have you tried?