Like sapphires, rubies are also members of the corundum mineral species. The distinguishing characteristic of a ruby is its red color. Rubies receive their red color from high levels of the trace element chromium. Color is the most important factor in determining a ruby’s quality. Most fine rubies have a medium to medium-dark tone. A ruby should not be so dark that its color is obscured, nor so light that its color appears indistinct. Rubies have a unique characteristic: they fluoresce red in daylight. Fluorescence refers to an object’s ability to emit visible light when it is exposed to radiant energy. Burmese rubies which form in marble tend to have fluorescence. Certain rubies can also exhibit a property called asterism. A six rayed ‘star’ is most often displayed in rubies, but a twelve-ray star can occur and is highly prized.