Ruby is red corundum, all other color varieties of corundum being referred to as sapphire. The ruby color range includes pinkish, purplish, orangey, and brownish red depending on the chromium and iron content of the stone. The trace mineral content tends to vary with the geologic formation which produced the ruby, so original place designations such as Burmese and Thai have come in later years to be sometimes used in describing color.
Most authorities expect a medium to medium dark color tone in a ruby, naming stones lighter than this, pink sapphire but there is no general agreement exactly where the line is to be drawn. The old joke about questionable stones goes: "Whether it's a ruby or a pink sapphire depends on whether you're the buyer or the seller."
All corundum gems including ruby have a long history of enhancement. Unless the seller specifically states the stone is unheated, you should assume that some kind of heat treatment has been used. Usually high temperature heating and controlled cooling is done to clarify the stones, especially by dissolving "silk" (rutile); but it can also improve tone and saturation of color. Such treatments can only be detected in stones whose residual inclusions show signs of heat stress; truly clean stones will give no clues and cannot be verified as natural color. The general view at present seems to be that simple heating, being indistinguishable from Nature's own heating processes, and stable, is acceptable as long as it is disclosed. For this reason such enhancement does not radically lower the value of ruby gems. Not so for other more recently invented treatments such as diffusion coloring, or polymer or glass filling.
Corundum was first synthesized in the early 1900's by a simple flame fusion process. Many jewelers and gemologists have had the unpleasant task of telling the proud heir that Grandmother's treasured ruby ring or brooch contains a flame fusion stone and has a lot more sentimental than commercial value. More complex synthesis processes have been developed in recent years. These so closely simulate natural formation conditions that colors and even inclusions look extremely natural and such stones are difficult for all but the most highly skilled professionals to identify as man-made.
Ruby is hard (9) and tough, making it a superb jewelry stone. (Of course, a heavily included or fractured stone will be less stable.) For reasonably clean stones, no special wear or care precautions are necessary. Ruby shows pleochroism which means that the color varies with the direction of viewing. Most stones show purplish red and orangey red, although the presence or absence of trace minerals can dampen either of these. The overall color can often, but not always, give a clue to a stone's geographic origin, with Burmese stones tending to purplish red colors and Thai stones appearing more brownish red. In addition, many rubies will fluoresce in long or short wave UV and this property can often be used to help identify a stone's geographic origin. Burmese rubies often fluoresce so strongly that the effect is noticeable even in sunlight, such stones seem literally to glow, and are greatly admired. Thai stones generally lack this property. Although Asia has historically been the major producer of ruby gems, there are many other sources including the USA, Australia, and most recently Africa.
Ruby rough of lower quality is used in great quantities to make beads, carvings, and other ornamental objects. The silk, which is so common in corundum, can, if sufficiently abundant, and precisely arranged, lead to asterism. With proper cutting, this creates star rubies. Today there are heating and diffusion processes that can increase the rutile content and improve such gems. Synthetic star corundums were very popular in the 1950's under the trade name "Linde Stars" and are still under production.
Few other gems have as much myth, lore and romance surrounding them, with one of the chief attractions being the protection from misfortune and bad health rubies were believed to afford their lucky owners. As the science of gemology developed it became known that many historically important "rubies" such as the famed Black Prince's Ruby of the British Crown Jewels, were actually other red gems, most often red spinels.
Rubies are the most valuable members of the corundum family. Large gem quality rubies can be more valuable than comparably sized diamonds and are certainly rarer. There is a relative abundance of smaller, (1-3 carat,) blue sapphires compared to the scarcity of even small gem quality rubies, making even these smaller stones relatively high in value.
Stones of Burmese origin generally command the highest prices. The vast majority of rubies are "native cut" in the country of origin. High value ruby rough is tightly controlled and rarely makes its way to custom cutters. Occasionally, such native stones are recut to custom proportions, albeit at a loss of weight and diameter. Custom cut and recut stones are usually more per carat, and my own bias is that they are worth it.
Sinkankas and Miller in the Standard Catalog of Gem Values, 2nd. Ed. list a wide range of wholesale prices for faceted gem rubies. Prices are dependent on origin, color, size, and clarity: from a low of $100 to $15,000/ ct maximum.
Burmese stones in 1/2 to 1 ct sizes with slightly purplish red color and light inclusions range from $300 to $3000/ ct, for example. The price survey done by the International Gem Society reports that clean, top color gems in the 1/2 to 1 ct size range are being sold, retail, on the Internet with a range of $1000 - $3000/ct.