Corundum is a crystalline form of aluminum OXIDE, which is the second hardest mineral on the Mohs scale. EMERY, with five letters, is a type of corundum that is used as an abrasive. SAPPHIRE, with eight letters, is a type of corundum that is known for its blue color and is often used in jewelry. RUBY, with four letters, is a type of corundum that is known for its red color and is also often used in jewelry. OXIDE, with five letters, is a chemical compound that is composed of oxygen and another element, and corundum is an OXIDE of aluminum.