Ruby is the gem quality of the mineral corundum, one of the most durable minerals which exists, a crystalline form of aluminium oxide. Corundum has a hardness of 9 on the Mohs scale and is also extremely tough. In its common form, it is even used as an abrasive.

Choosing a Ruby

The most important factor in the value of a ruby is color. The top qualities are as red as you can imagine: a saturated pure spectral hue without any overtones of brown or blue. The intensity of color of a fine ruby is like a glowing coal, probably one of the most intensely colored substances.

All colors of corundum except red are known as sapphires, which has created controversy about where ruby ends and sapphire begins, particularly in pink shades of corundum. The lighter shades of the reddish hues of corundum should be included in the category of ruby.

After color, the other factors which influence the value of a ruby are clarity, cut, and size. Rubies that are perfectly transparent, with no tiny flaws, are more valuable than those with inclusions which are visible to the eye. Cut can make a big difference in how attractive and lively a ruby appears to the eye. A well-cut stone should reflect back light evenly across the surface without a dark or
washed-out area in the center that can result from a stone that is too deep or shallow. The shape should also be symmetrical and there should not be any nicks or scratches in the polish. Rubies and other gemstones are sold per carat, a unit of weight equal to one-fifth of a gram. Larger rubies, because they are more rare, will cost more per carat than smaller stones of the same quality.

Ruby sometimes displays a three-ray, six-point star. These star rubies are cut in a smooth domed cabochon cut to display the effect. The star is best visible when illuminated with a single light source: it moves across the stone as the light moves. This effect, called asterism, is caused by light reflecting off tiny rutile needles, called "silk," which are oriented along the crystal faces.

The value of star rubies and sapphires are influenced by two things: the intensity and attractiveness of the body color and the strength and sharpness of the star. All six legs should be straight and equally prominent. Star rubies rarely have the combination of a fine translucent or transparent color and a sharp prominent star. These gems are valuable and expensive.