05.09.2022 Heat Treatment of Gemstones
Heat treatment of gemstones is a widely used method to “improve” the quality of gemstones.
Today, the vast majority of all gemstones sold are treated, because only about 2% of gemstones on the world market have “gemstone quality” without any treatment. The more treatments a stone has undergone, or the more intensive it was, the lower its retail value, because untreated gemstones are the most valuable.
However, some particularly desirable color varieties are naturally occurring very rare, so there is also a demand for certain heated color gemstones.
How does the heating of gemstones work?
Exposing gemstones to heat has a long tradition. For thousands of years, gemstones have been heated in fire or ash to change their color. Nowadays, so-called muffle kilns are used for heating, which are generally used for ashing, annealing and heat treatment of workpieces. These are heated to a temperature between 300 and 1900 °C.
Depending on the mineral and temperature, the heat treatment changes the color or the stone becomes purer. That’s because inclusions melt and diffuse, like for example rutile inclusions in rubies and sapphires. In addition to the temperature, the duration of the heat treatment, the atmosphere during heating and the chemical structure of the surrounding or co-fired materials also influence the final color result.
Improve the color and purity of gemstones through heating
Basically, there are low-temperature treatments used to change the color of some minerals (up to about 1,100 °C) and high-temperature treatments (up to 1,900 °C) to trigger color changes and also to remove inclusions.
For rubies, heat treatments above 1,100 °C remove brown, purple, or purplish hues that are undesirable in pure red. Firing at temperatures above 1,450 °C also dissolves fine rutile inclusions, known as “rutile silk”.
Sapphires that are light blue or milky become intensely blue by firing at 1,250 to 1,700 °C; colorless to pale yellow ones, in turn, become intensely yellow by heat treatments. Pink sapphires are fired to achieve the coveted orange-pink color variety “Padparadscha.”
Low-temperature treatments are used on aquamarines, tourmalines, amethysts, and morganites to enhance their color. Thus, pink morganites are heated orange beryls. In the case of aquamarines, amethysts and tourmalines, the color is refined. For example, in naturally occurring amethysts, the color distribution is very often irregular. If you expose them to a temperature of about 300 °C, the color becomes more regular and fuller. However, above temperatures of 470 °C the color of the normally purple amethysts changes – it turns to yellow, the gems become citrines.
Can you tell heated gemstones from untreated ones?
The colors resulting from heat treatments are stable, i.e. they do not change even due to UV irradiation, temperature fluctuations or other factors.
In high-temperature treatments, the so-called blast halos can be an indication that heat treatment has occurred: a bubble-like looking halo forms around the melted inclusion. However, these can also occur naturally in stones and form during the formation process.
Low temperature treatments are more difficult to detect. Nevertheless, well-equipped gemological laboratories today can also determine whether a gemstone has been subjected to a low-temperature treatment.
The “Confederation Internationale de la Bijouterie, Joallerie, Ofevrie, des Diamantes, Perles et Pierres (CIBJO)” declares in its trade rules that the treatment of gemstones by heat should be indicated when trading such stones. Common terms for this are “heat treated”, “heat enhanced” or “heated”. However, these rules do not have the force of law; they are to be understood as commercial practice.
The price difference between a heat treated gem and a natural gemstone can be huge. When buying gemstones for investment purposes, always look for certificates from an independent gemmological laboratory! These always include an indication of whether a stone is natural color, untreated or heat treated.