I have had for a while a rectangular, stone-like file, it is roughly 10cm long, and about 6mm each side square, it has snapped in two pieces, I am attaching a photo of the smaller piece (have probably lost the other), it is very fine and have used it to gently sand down bits and pieces like say epoxy coated copper wires. I would like to buy a new one, but I cannot find it I do not even know what it's called.