Rubies are gem corundums with a dominant red color. The color can range from orangy red to purplish red or brownish red. The most desirable color range is a pure vibrant red to a slightly purplish red.

The red color of ruby is produced by the presence of chromium in the gem. A small trace of chromium will produce a pink color. To be considered a ruby, there must be enough chromium to give the gem a distinctly red color.

Finally, rubies must have a combination of color and clarity that makes them an attractive gem. Opaque pieces of corundum with just a hint of red color are not rubies – they are common corundum.