Rubies can command the highest per-carat price of any colored stone. This makes ruby one of the most important gems in the colored stone market. In its purest form, the mineral corundum is colorless. Trace elements that become part of the mineral’s crystal structure cause variations in its color. Chromium is the trace element that causes ruby’s red color.
Color – Color is the most significant factor affecting a ruby’s value: Fine gems are a pure, vibrant red to slightly purplish red.
Clarity – If a ruby’s inclusions affect its transparency or brilliance they reduce the gem’s value significantly.
Cut – Rubies are commonly fashioned as mixed cuts, which have brilliant-cut crowns and step-cut pavilions.
Carat weight – Fine-quality rubies over one carat are very rare and the price goes up significantly as size increases.