Classification of Rubies

Chemical Classification

• Rubies are a red variety of the mineral species corundum (an aluminum oxide). The chemical formula for corundum is Al2 O3. Ruby occurs when an amount of chromium oxide replaces some of the aluminum, making the chemical formula for ruby Al2 O3:Cr. When tested under ultraviolet (UV) light ruby fluoresces red.

Physical Features

• The internal crystalline structure of a gem determines its shape. Different crystalline structures have been classified and are called crystal systems. Rubies are classified under the hexagonal system. This means that rubies tend to form shapes with six sides

  The hardness of a ruby is rated using the Mohs scale of hardness, a relative scale on which a diamond rates 10. On this scale a ruby is rated a 9, meaning it is a hard gem. Though hard, rubies are brittle. When a ruby does break, the way it fractures is called conchoidal, due to the fractured surface appearing either concave or convex, like a shell.

  Rubies can have inclusions. The most common inclusions include spinel, zircon and mica, and rutile.

Color, Cut and Clarity Value Classification

• In the gem business, a ruby is classified according to the depth of its color after it has been cut. The deeper the red, with a hint of blue, and with little to no brown or stripes, the higher its value is. Other valuing classifications include cut (in which symmetry, facet placement and even polish are important) and clarity (a quality of being pure, transparent and clear).

  Even though a clear ruby is valued a star ruby is classified with a greater value. This type of ruby occurs when rutile inclusions (look like needles) line up at sixty degrees to each other. A star-like shape of reflected light occurs in the gem, an effect called asterism.

Location

• Location is also a type of classification of rubies, of which there are several main types: the Thai ruby, the African ruby, the Ceylon ruby and the Oriental ruby (also called the Burma ruby or the Burmese ruby). These rubies from each region tend to have certain characteristics idiosyncratic to the area they are found in.

Historical Classification

• Back in Roman times, the ruby was confused with other red stones such as garnet and spinel, classing all of them as the same stone. This continued until 1800. Rubies were thought also to have medicinal properties, bestowing such things as wealth, invulnerability and wisdom as well as romance.