Silicon Carbide, or Carborundum is a man-made material that is generally used for abrasive materials such as grinding wheels. The material is very brittle and sharp, and has a good electrical and thermal conductivity. The specimen you have is from smokestack slag. Chemical composition is very complex, containing many different materials.

Silocon Carbide samples are a minimum of 2" on their longest size. Comes in a cardboard gift box.