Gem-quality corundum, generally known as ruby and sapphire, is the second hardest gemstone following diamond. It is widely accepted in the gemstone market that gem-quality corundum is one of the most valuable precious stones that is available in a wide variety of color.
Because a naturally occurring high-gem-quality corundum is rather rare or difficult to find in nature, a number of methods have been invented to improve quality of stones to the level that is preferable in the market (approximately from 1,000 pieces of colored stones, only 1 piece is untreated). Enhancement is considered a necessary process for an adequate supply of high-quality gemstones in the market, as well as a key factor in raising the price.
Colored stones enhancement in various methods is a long traditional practice in Thailand’s gemstone industry. The most popular method is heat treatment which was discovered in Chanthaburi, Thailand, around forty years ago. There was a great fire which lasted for three days in Chanthaburi. After the fire was extinguished, a gem dealer surprisingly found that his colored stones gained much better quality. From this discovery, the heat treatment technique has been developed continually and becomes Thailand’s strength and competitive edge over its competitors until today.
Traditional Heat Treatment
The method is carried out by simple heating of stone in a controlled furnace at high temperature appropriate for each type of raw material. This method was used in the early day of treatment and is still a common practice at the present time. This is because the color after treatment is still caused by the chemical nature of the stone itself, hence the technique is well-accepted in the gem market. The fact that Thai gemstone heat treatment specialists have used their local wisdom, trial and error, and continuous effort. As a result they are able to heat-treat various types of raw materials from different localities in different types of furnaces to their optimum conditions. Thai gemstone heat treatment experts have developed both coke and oil “home-made” furnaces that can produce reducing atmosphere suitable for treatment of blue sapphire. They also develop “home-made” electric-heating-coil furnace as well as propane-gas-oxygen furnace that can produce oxidizing atmosphere appropriate for treatment of ruby and yellow sapphire.
Flux-Enhanced Heat Treatment
This method is carried out by heating stone at high temperatures with the addition of flux materials, such as borax and silica, in the crucible. Since the flux materials will be melted below the melting temperature of stones, they form a glassy material that can heal fractures and fill cavities, which in turn prevent the stones from cracking while heating up and cooling down. Moreover, the use of flux not only prevents stone from falling apart during heat-treatment but it can also disguise the presence of fractures (due to its higher refractive index) which make the stone clearer.
Surface Color Diffusion Heat Treatment
This heat treatment method is used for coloring of colorless sapphire as well as gemstones undergone simple heat treatment but are still colorless or have too light color. The method makes those gemstones turn into blue or red by heating them with coloring agents. To create the blue color, titanium compounds are usually employed. To create the red color, chromium compounds are usually used. This heat treatment method allows the coloring agent to diffuse from the gemstone surface into the gemstone structure. The rate of diffusion of titanium or chromium into gemstones is extremely slow although the heat treatment is conducted at high temperatures for a long period of time. Therefore, this heating technique is usually carried out on the cut stones, which usually possess very thin color rim at the surface only.
Deep Color Diffusion Heat Treatment
This heat treatment method is invented by diffusing some light elements such as beryllium into the gemstone structure from outside. The heat treatment through beryllium diffusion allows the change of gemstone color from colorless or greenish blue to yellow which cannot be done by simple method. It can also change the color of sapphire from pink to orange, or to pink-orange (so-called ‘padparadscha-like’ sapphire).
Source : Gem and Jewelry Information Center The Gem and Jewelry Institute of Thailand (Public Organization) February 2016