A sphere made from pure Silicon metal.

I bought this piece from GeoFossils when they came up for a Rock and Gem event in Edinburgh. 

As you can see from the second photo below, I was convinced the whole thing would cleave apart at any part of the process but luckily, it held together and the cracks actually compliment it quite well. It was a very bizarre material to work with; it conducts heat very quickly, so instantly got extremely hot while polishing it and extremely cold when washing it.