Unworked mas of silicon carbide (carborundum) crystals.
© Science Museum / Science & Society Picture Library
Silicon carbide (SiC) is held together by bonds resembling those which hold a diamond together, so that it forms a giant molecule in which every atom is directly linked to four other atoms disposed tetrahedrally around it. These links are very hard to break, so silicon carbide, like diamond, is a very hard substance. It is often used as an abrasive.