These samples of corundum are displayed in the Smithsonian Museum of Natural History. Corundum is a oxide mineral of aluminum with the composition Al2O3. The sample at left is about 6 cm across and is from Franklin, New Jersey.
Pure corundum is colorless, but the addition of a small impurity of chromic oxide can make the precious stone ruby (red) and the addition of titanium oxide can make sapphire (blue and other colors). From the Museum of Natural History description "Corundum rarely has the clarity or richness of color to be a gemstone. When it does, the difference between a ruby and a sapphire is just a tiny bit of impurity."
Corundum and impure corundum called emery are used in making abrasive cloths and wheels. Corundum is second only to diamond in mineral hardness. It measures 9 on the Mohs hardness scale, compared to 10 for diamond.