Sometimes water is a part of the chemical formula. The characteristics of oxides vary greatly. On the Mohs scale of hardness, they can very from 1, some hematites, to 9, corundum. The colors vary from drab black, gray, and brown to very colorful shades in rubies and sapphires.
Most basic oxides form deep within the earth s crust from the solidification of magma.
The most common oxides are formed from the alteration of sulfide minerals.