Centuries ago, someone stumbled upon the magical effect of applying heat to gemstones. High heat, such as that from a charcoal fire, can make a bland looking gemstone change its color into something spectacular. If this fortunate technique were not discovered, there would be very few affordable gemstones of good color in the market.
Heat treatment is considered a natural type of enhancement as it is a continuation of the processes that occur in the earth when the stone was originally formed. During treatment, the stone is heated to very high temperatures (approximately 1600 Centigrade) causing inclusions, chemical elements, and other impurities to reform themselves and change the color of the stone. This color change may result either in the stone being darker, lighter, more intense, or of a different color. An example of this is the dissolving of rutile silk inclusions in blue sapphires, which improves both clarity and color. This heat treatment is permanent and irreversible.
Another example is ruby. This a stone that is commonly heat treated. Only the most valuable and expensive rubies possessing the richest colors are not heat-treated. Ruby is heated almost to its melting point, allowing the aluminum oxide in the stone to reform, creating a new crystal structure. This allows the chromium in the stone to combine with different atoms, allowing for a better color of red. The same can apply to a type of sapphire known as gouda sapphires. These milky white sapphires turn blue, and account for many of the quality sapphires on today's market.
Detection of heat and diffusion treatment is possible because these treatments modify natural inclusions. The destruction of gas or fluid inclusions or the dissolving of mineral inclusions are clues to heat treatment. For gems that contained rutile needles, the needle margins may become diffuse. On rubies, inclusions may be found that are glassy in appearance. These are caused by borax-based substances that are used in the heat treatment process.
However, it is usually more difficult to know if a stone has not been treated, in other words, has the stone never been treated? Gemologists can examine the inner workings of the stone and study the inclusions for signs of heat treatment. For example, if the stone has been treated, tiny inclusions such as small crystals will melt during the heat treatment process. A gemologist can easily see this using a microscope.
Centuries ago, men sitting in front of charcoal fires were the first practitioners of this art. They would blow air through pieces of bamboo into their glowing charcoal where a few stones were placed in an attempt to coax some new colors into their stones. Today, the technology is much more sophisticated, with professionals using large computer controlled electric furnaces. The old, crude methods are gone, but the result is still the same: drab gemstone are turned into something beautiful. This allows us all to have the chance to own a colored gemstone that we can be proud to show.
Amethyst, citrine, ametrine, aquamarine, tourmaline, topaz, light green tourmaline, sapphire, ruby, tanzanite, and blue zircon are typically color-enhanced by heat gemstones. Here is a full list of the more commonly heated stones and how heat treatment enhances them.