Ruby or spinel, a combination of alumina and magnesia, scratches quartz, is scratched by corundum, and is usually of a more or less vivid red; lapidaries distinguish two varieties - the spinel ruby, of the deepest colour, next in value to the red sapphire or oriental ruby, for which it is often substituted, and the balas ruby, more of a rose or pale red hue, its tint approaching that of the burnt topaz.

Spinel proper, when closely examined, always appears in broad daylight of a rosy shade. Under similar conditions the red sapphire, or oriental ruby, shows a very decided purple violet tint. It is this that is described in the old inventories under the name of ruby.