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 colored stone. This makes ruby one of the most important gems in the colored stone market.
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, which ranges from an orangy red to a purplish red.
The most renowned rubies, like those from Myanmar, the Himalayas, and northern Vietnam, typically form in marble.
Source: GIA Gem Encyclopedia