In our newsletter this month:Understanding Heat Treatment
As part of our continuing series of articles on understanding gem treatments, this month we look at heat treatment.
The heating of gemstones to enhance their color and clarity goes back centuries, probably beginning with the lucky fellow who accidentally dropped his valuable gem into a fire, only to discover the next day that the color had improved. It must have seemed like a kind of alchemy. Even today heat treatment of gemstones is a secret art that is known to very few.
Depending on the type of gem and the heating conditions, heat treatment can lighten or darken the color of a gem, remove certain color tones or improve clarity. Heat treatment is most commonly used with sapphire and ruby, but there are a number of other gem varieties that are enhanced by heat. These include zircon (blue and yellow), apatite, morganite, tanzanite and blue topaz (in combination with irradiation).
Heat treatment works by changing the nature of impurities in the gem material. These impurities are usually responsible for the color we associate with the gem. For example, a ruby containing a mere 1% or less of chromium will appear pink or red, but in the absence of chromium the pure corundum will be completely colorless.
Since the impurities in the stone typically react to heat at a lower temperature than the gem material, heat can be used to change the chemical state of the impurity without affecting the base gem material.
In some cases, heat treatment will improve the depth of color because heat causes dissolution of inclusions and diffusion of impurities (especially titanium from rutile inclusions) into the surrounding corundum. Since fine inclusions cause some stones to look cloudy, heat treatment that dissolves the inclusions may also improve the clarity of the stone.
The recipes for 'burning' gems are carefully guarded and there are many variables involved. Temperature is only one variable. Gems may be heated at temperatures as low as 100 degrees centigrade and as high as 2000 degrees (approaching the melting point of corundum). The amount of oxygen in the environment is also important; some gems are heated in a reducing (oxygen-free) atmosphere and others in an oxidizing atmosphere.
Heat treatment can generally be detected only by trained gemologists who look for changes in characteristic inclusions in gem material like corundum. On the basis of these observations, gemological labs will issue identification certificates that state "evidence of heat treatment is present" or words to that effect.
It is generally agreed that simple heat treatment of gemstones is stable and permanent. This is not surprising since the heat treatment mimics natural processes involved in the formation of gemstones in the earth. Since the success of heat treatment depends on the different melting points of impurities and the base gem material, heat treatment should not affect the gemstone material itself.New in Gems
Our buyers are always busy searching for the best value gems for our customers. Here are some of the excellent buys we've made in the last few weeks:
Each month our staff select some of their favorite gemstones from our inventory. This month we feature some of the finest of our recent acquisitions. Click on the photos to go to the product pages.
Gemstones Worth Knowing
Each month we focus on one of the lesser-known gemstones. This month's featured stone is andesine labradorite.
There are fashions and trends in the gemstone world, and andesine is an unusual gem that has drawn tremendous interest in the past year. Gemologists have been particularly intrigued because red feldspar is extremely rare, and different stories have been told about the origin of this material. The original material in the market was identified as coming from the Democratic Republic of the Congo, but recently, similar feldspar said to be from Tibet has come on the market.
Adding to the slightly mysterious nature of this gem, it is variously known as andesine and labradorite, or as many people now call it, andesine labradorite. It is found in a range of colors, including red, yellow, champagne and green, but the red is the most valuable.
To see all our stock in this intriguing gemstone click here.Customer Questions
Every month we answer questions of general interest from our customers. Please feel free to send your questions to email@example.com, with "newsletter question" in the subject line.
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Happy gem hunting!
Your friends at GemSelect
- First Published: October-01-2007
- Last Updated: November-04-2014
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