In its purest form, the mineral corundum is colourless. Trace elements that become part of the mineral’s crystal structure cause variations in its colour. Chromium is the trace element that causes ruby’s red, which ranges from an orangey red to a purplish red.
The strength of ruby’s red depends on how much chromium is present—the more chromium, the stronger the red colour. Chromium can also cause fluorescence, which adds to the intensity of the red colour.
The most renowned rubies, like those from Myanmar, the Himalayas and northern Vietnam, typically form in marble. They’re found in layers that are distributed irregularly within the surrounding marble. Marble forms as part of the metamorphic (rock-altering) process, when heat and pressure from mountain formation act on existing limestone deposits.
Marble has low iron content, so the rubies that originate in marble (called “marble-hosted” by gemmologists) lack iron. Because of this, many have an intense red colour.
In addition, rubies found in marble typically fluoresce red under ultraviolet light—even the ultraviolet light in sunlight. Fluorescence can make a ruby’s colour even more intense and increase its value.
In other locations, rubies can be found in basalt rocks. Rubies from these sources can have higher iron content, which can make the rubies darker and less intense in colour. Higher iron content in the chemical makeup of a ruby can also mask the red fluorescence, eliminating that extra glow of red colour seen in marble-hosted rubies.