Heat Treatment of Corundum
In absolutely natural condition only the very fewest gemstones are used for jewellery purposes. In addition to the actual cutting process, gemstones are in many cases subjected to other processes in order to change colour, clarity, sparkle or also the grindability in a positive sense.
Such modifications are called “treatments” or “artificial property changes” and the treatment of gemstones causes a larger number of rough gemstones to meet the requirements of the market, i.e. the supply of attractive material is increased, leading to significantly improved market prospects. Without this practice, we would see far less sapphires on the market with similar good clarity or colour, and at far higher carat prices due to limited supply, hence making sapphires more accessible and affordable. Unaffected by this is the low supply of natural material used for jewellery.
Only about 1-2% of discovered sapphires are of gem quality without treatment.
The thermal treatment (burning / heating) of rubies and sapphires is a standard treatment in today’s market. What most people are unaware about is that the heating process helps optimize colour and clarity of gemstones.
Here is a visual example of what effects heat treatment can have. The Art of heating is very complex and requires expertise in order to bring out the best in every stone.
These sapphires were cut before the heat treatment to demonstrate the possible colour changes (left half before, right half after the heat treatment).
In this treatment, rubies and sapphires are usually heated above 1200°C. This changes the value of certain colour-effective trace elements, which explain the colour optimisations. Here it is true that not every stone can be changed to optimum colours by temperature treatment, i.e. the raw material for this is also limited. Furthermore, dulling inclusions can also dissolve, which leads to an improvement in clarity.
Natural, untreated corundum without heating have a prefix “no indications of heating” or “NH”. The prices per carat are significantly higher for untreated corundums because they are even rarer and face an even higher price development over time, especially for larger carat weights above two, five or even ten carat. Although they are no more beautiful, but their rarity makes them highly collectable and prices are set at a premium, sometimes fetching triple the price paid for an equivalent heated sapphire.
Through our exceptional network we have access to unheated sapphires and real rarities. All other forms of treatments (chemical optimization, irradiation, dying, surface diffusion, etc.) we entirely reject because we stand for the natural unaffected beauty of Ceylon sapphires and gems.
The temperature treatment of corundum can often be demonstrated by changes in inclusion diagnostics. Spectroscopic examinations in gemmological laboratories provide further possibilities for detection.
Technical aspects of heat treatment to sapphires and rubies
“Heat enhancement is applied to sapphires or rubies is to improve colour and/or clarity.”
How to identify heat treatments? The most common changes
Loosened rutile needles identify this sapphire as heat-treated
These mineral inclusions are fused during heat treatment
Often melted mineral inclusions resemble "snowballs"
Stained colour zonings indicate heat treatment
With heated blue sapphires, the colour zoning often follows former rutile needles
Heat treatment can often cause burst cracks to form around inclusions
Burst cracks often resemble "island atolls" in their structure
During the temperature treatment, even the surface of an already cut stone can melt
Other Treatments to Sapphires
A sapphire can be listed as “natural” simply because it is not lab created. However, “natural” does not mean the sapphire has not undergone any alterations. The key term is “untreated,” meaning a sapphire is natural (not lab created) and has not been subject to any chemical or heat treatments. These processes may increase the stone’s beauty, but not its value.
During the past two centuries, scientists have derived an assortment of methods to create gem crystals in the laboratory. Buyers have to be careful when purchasing natural corundums as synthesized gems of the same crystals are available on the market. Not correctly declared synthetic gems as syntheses is strictly illegal. The U.S. Federal Trade Commission (FTC), the International Coloured Gemstone Association (ICA) and the World Jewellery Confederation (CIBJO) have issued strict procedures that disclose the synthetic gemstone’s origin throughout the distribution channel at the time of sale, from the manufacturer to the consumer. Therefore it is important knowing the exact source, having trustful business partners and assuring the authenticity by gem lab reports for valuable gems.
Let’s have a look at ``Syntheses``
Today, rubies and sapphires are also produced synthetically in large quantities unfortunately. The most important process is the so-called Verneuil process, with which corundum can be produced quickly and inexpensively. The identification of syntheses is carried out in all cases, also with other synthesis methods, by microscopic examinations by an experienced gemmologist.
Curved colour zonings diagnose this sapphire as synthetic.
Curved growth strips are also typical characteristics of Verneuil syntheses.