Stone or rock, is a naturally occurring solid aggregate of one or more minerals or mineraloids. They have been used by mankind throughout history. From the Stone Age, stones have been used for tools. The minerals and metals found in stones have been essential to human civilization.
Spinel is the magnesium aluminum member of the larger spinel group of minerals. Its hardness is 8, its specific gravity is 3.5–4.1, and it is transparent to opaque with a vitreous to dull luster. It may be colorless, but is usually various shades of red, blue, green, yellow, brown, or black.