Silicon carbide
The carbides of silicon and boron are described as "covalent carbides", although virtually all compounds of carbon exhibit some covalent character. Silicon carbide has two similar crystalline forms, which are both related to the diamond structure. Boron carbide, B4C, on the other hand, has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect boron carbide is similar to the boron rich borides. Both silicon carbide (also known as "carborundum") and boron carbide are very hard materials and refractory. Both materials are important industrially. Boron also forms other covalent carbides, e.g. B25C.