Ruby is the red variety of the mineral corundum. Chromium causes its red color.
In more detail
Ruby has the same refractive index (1.762 to 1.770), hardness (9) and specific gravity (4.00) as sapphire. Red must be the dominant hue before GIA calls a stone ruby; a stone too light may be pink sapphire. Corundum has excellent toughness and no cleavage, so ruby suits daily wear.
On a grading report
Labs identify it as (natural) corundum, species or variety ruby. Any treatment, such as indications of heating or glass in fissures, goes under Further information or Comments.
Related terms
- Corundum: Corundum is the mineral species made only of aluminum and oxygen. Its red variety is ruby, and all its other colors are sapphire.
- Sapphire: Sapphire is corundum of any color other than red. The word alone means a blue stone; other colors take a color word, such as yellow sapphire.
- Pink sapphire: Pink sapphire is corundum whose color is pink rather than red. Chromium causes both its pink and the red of ruby.
- Pigeon's blood: Pigeon's blood is a trade name for a ruby of intense, saturated, even red. It has no single agreed standard.
- Heat treatment: Heat treatment is controlled heating of a gem to change its color or clarity.
- Silk: Silk is a mass of fine, crossing needle inclusions, usually rutile, in ruby and sapphire.