Natural rubies are gemstones with colours varying from light red to dark red. Like sapphires, rubies are of the corundum mineral species and they are sometimes referred to as red sapphires. The presence of the element chromium is what gives rubies their red colour. Gems in the corundum species are extremely hard stones, second only to diamonds. Historically, rubies have been mined predominantly in Asia but are found in countries around the globe.
As with diamonds, the cost of a ruby depends on its 4C profile—colour or hue, cut, clarity, carat weight, and country of origin—as well as a criterion called “crystal” or transparency. In rubies the primary hue is red. A ruby may also exhibit secondary hues including orange, purple, violet, and pink. The primary colour of high quality rubies is a vivid medium-dark toned red. A purple secondary hue is desirable, because it enhances the richness of the red colour.
Pigeon blood-red rubies are the most vivid and most valuable and command a huge premium over similar quality rubies. After colour, discriminating buyers seek clarity. Cut and carat weight also have a bearing on the price of a ruby.
All natural rubies have imperfections such as colour impurities and inclusions of rutile needles known as “silk.” These needle inclusions are how professionals distinguish natural rubies from synthetics; a ruby without rutiles indicates the stone may have been treated. Like blue star sapphires, some rubies have a three or six-point asterism, or “star,” resulting from intersecting needle-like inclusions. Star rubies are rare and desirable, and their inclusions do not necessarily impair the quality of the stone unless they interfere with transparency or are located at the centre of the table. Often inclusions are valued for statements of individuality and proof of authenticity and natural origin.
As with all gemstones, a masterful cut is a key factor contributing to the beauty of a ruby. In general, a ruby with virtually no inclusions, vivid colour, and of large size can surpass the value of a diamond in the same category.