Ruby
Ruby is the most valuable variety of the corundum mineral species, which also includes sapphire. Rubies can command the highest per-carat price of any coloured stone. This makes ruby one of the most important gems in the coloured stone market. Chromium is the trace element that causes ruby’s red. The strength of ruby’s red depends on how much chromium is present. Ruby's have a hardness of 9 on the MOH's scale of hardness.
