When two oxygens are shared with adjacent tetrahedra, chains of SiO4 tetrahedra are formed as shown at left. When each SiO4 unit shares three of its oxygens with adjacent SiO4 tetrahedra, two-dimensional sheets of SiO4 tetrahedra are formed as shown in the diagram at right. The top part of the diagram shows that the sheets contain 6-membered rings, where each of the 6 members is a silicate tetrahedron. Hardness - How resistant they are to scratching. Color can be changed dramatically by the addition of only a few tenths of a percent of a trace element such as Fe, Ti or Cr.
Pure corundum is colorless.