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The Heat Treatment of Ruby & Sapphire

A lot of people are familiar with two kinds of gemstones: natural ones and their man-made versions. Relatively less of the community has any stock knowledge on treated gemstones. Many people however have probably already purchased some, given that a majority of the top three colored stone varieties undergo common treatment procedures before heading off to retail.

Now, while both natural and synthetic stones can each be "enhanced" by artificial techniques, it's usually the natural ones that undertake the most treatment procedures. This is so manufacturers can increase their apparent qualities and characteristics, making them more saleable to the consumer population.

Let's look at one of the more common treatments and how it affects certain gemstones.

Probably the most notable type of enhancement would be heat treatment. This process can be done with or without additives, but sometimes evolves into another type of treatment depending on the type of substances added during the process.

Heat treatment alone is used on a variety of gemstones, including most kinds of corundum. Applying controlled heat to blue sapphire for example, can either lighten or deepen its color. The result depends on whether the actual procedure takes place in a reducing (oxygen-free) or oxidizing environment. The first has been observed to cause light or pale blue stones to turn darker, while the latter causes dark inky stones to mellow into a more medium blue color. Temperatures for this would be set at about 1600 degrees centigrade, but different settings are also used to incur a variety of color altering results.

Heat Treatment at Safe Temperatures Generally Improves Color in Rubies and Sapphires. Occasionally Some Inclusions Can Become Slightly Altered in the Process, Giving Gemologists a Quick Way to Identify Treatment.

Heating blue sapphires at certain higher temperatures can also affect its clarity. The presence and reaction of titanium is what allows this to happen in such procedures. Applying heat of about 1800 to 1900 degrees centigrade for a long period of time can cause the titanium oxide content to precipitate as "silk" or tiny needles. These are what cause the "velvety" visual effect in many high-end stones.

On the other hand, applying that same temperature for only a brief period of time can do the opposite and instead "dissolve" silk and other inclusions by causing them to be reabsorbed into the stone's chemical make-up. This can let a stone look more transparent as a result.

Heat treating ruby is usually done in an oxidizing environment to remove its purplish modifying hue. The temperature used here is often about 1800 degrees centigrade. Sometimes heating is also done to break down silk in some specific stones. 

Some rubies are heated with a flux substance such as borax, to slightly meld or heal fractures. Gemological testing can also reveal this, and describes the treatment as a variation of the phrase "heated with residue". The apparent clarity is improved this way, and the stone altogether would probably look more clear compared to before.

Heat treatment is sometimes done with the addition of certain chemical elements. This often involves high temperatures in the hopes that the additives will permeate through the crystal's structure and induce an artificial color. 

We call this treatment "lattice diffusion" and while it's not as common as normal heat treatment, products of this enhancement are generally seen in the trade today.

Pale blue sapphire undergoes titanium diffusion to create a deeper darker blue color, while other corundum variety colors like that of Padparadscha can also be created using the diffusion of certain elements like Beryllium.

New ways of heating gemstones to produce color are being invented every day. Gemological knowledge is necessary to observe, detect and separate stones according to their corresponding identities so that proper disclosure and value assignments can take place in the industry. 

Today, about 95% of rubies and sapphires that make it to the jewellery trade are products of some form of heat treatment. It's one of the most accepted procedures in the industry as long as everything is met with openness and transparency.
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