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 word red is derived from the latin for ruby, ruber, which is derived from similar words in Persian, Hebrew, and Sanskrit. The intensity of color of a fine ruby is like a glowing coal, probably the most intensely colored substance our ancestors ever saw. It is no wonder they ascribed magical powers to these fires that burned perpetually and never extinguished themselves.

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. In 1991, the International Colored Gemstone Association ruled that 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.