Natural or heated sapphire: example of Padparadscha

Posted in FavouritePrecious objects

I was disappointed to learn from a trader leaving for Sri Lanka that the most sought-after pinkish-orange or orange-pink sapphires, called Padparadscha, were heated "most of the time". Slight heat treatment on site to boost the colour is therefore common practice to meet the demands of a rapidly growing market for coloured stones. These sapphires, heated at a low temperature of 1300° in traditional furnaces, are then buried in sand for slow cooling.

This particular colour evoking the sunset, is sought after for its rarity in the corundum family which sweeps the whole visible light spectrum; by convention the main red coloured stone is called ruby, the blue one is called sapphire, and for all the other colours, the dominant colour is specified, then possibly secondary colour (colourless sapphire, yellow-orange, pink, green...).

Exception for the Padparadscha (meaning lotus flower) which combines the two shades, with a dominant orange or pink, whose trade name comes from the history of the Tamil people beautifully told in Jacques David's latest novel "Padparadscha", published in 2017 by éditions jets d'encre.

 

Natural or treated stone?

Decree n°2002-65 of 14 January 2002 relating to the trade in gemstones and pearls in France has clearly established the distinction between "traditional lapidary practices" such as "heat treatment, provided that any heating residues on the surface do not cause a break in reflection visible under a 10x magnification magnifying glass".

Whereas, on the other hand, stones which have undergone "treatment by irradiation, laser, dye, surface diffusion, filling, possibly as a residue of a heat treatment, of colourless foreign matter solidified in external cavities which show breaks in reflection visible under a 10-fold magnifying glass, or by any other laboratory method which alters their appearance, colour or purity" must be marked "treated".

It's not easy to distinguish between the two! Depending on the temperature, heating is considered in some cases as a "traditional" heat treatment and in other cases it can change the colour and/or inclusions contained in the "treated" stone.

To put it plainly, either the stone is natural, without heating, or it has undergone a light "traditional" heating, or it has been treated by heating at 1600° combined with other chemical elements in order to modify the colour and make it pure.

The price

Considering that price differences are important for natural, unheated and untreated stone, up to 13 000 €/carat in the case of Padparadscha, the question of heating technology is important to know.

Synthetic stone or imitation?

Participating in a focus group with other experts not specialized in gemmology, I realized the possible confusion between the two cases.

Above all, it is essential to know the name of the mineral species, which is sometimes different from its commercial designation: the term "water sapphire" used in the lithotherapy world seems to me inappropriate; it is in fact a species called cordierite or iolite which presents an optical phenomenon called trichroism (3 distinct colours visible according to the orientation of the stone in relation to its optical axis) and which has nothing to do with sapphire (corundum family) which presents only a pleochroism (2 colours).

Synthetic sapphires: several processes

Then, on the jewellery market, there is an abundance of synthetic sapphires which have undergone, in addition, a treatment on their colour or purity in order to blur any identification of the methods used.

These synthetic sapphires are therefore stones produced in the laboratory from "schematically" the same chemical elements; certain characteristic inclusions are visible under a magnifying glass (Verneuil process developed at the end of the 19th century) which can be a source of confusion when selling them because it is indeed a "sapphire" to which must be added "synthetic" according to French legislation; man has produced a sapphire with the same main characteristics of the stone: its hardness, density for example, but in the laboratory.

In the end, very little difference to the naked eye, you might say.

The case of an imitation stone

On the other hand, imitation stones are natural stones or stones treated in order to confuse them with the original ... do you still follow me? For example, we could mount a red glass emerald-cut stone set with diamonds, sold as an imitation ruby. Glass is not a synthetic stone, it is another material that imitates the color of the ruby.

Conclusion

Colour remains an essential factor in the calculation of the price, but it is not the exclusive criterion; the quality of the cut with the respect of the proportions, in particular between the breech, the table, the facets, the purity, the colour zones, the polishing, all these criteria are taken into account for rare gems.

In my opinion, the natural inclusions characteristic of corundum, visible under a magnifying glass, remain the best evidence of traces of natural crystallization growth and are sometimes the source of proof of the origin of the deposit. Burma, Sri Lanka are the most sought-after origins, other deposits, Australia and Madagascar produce beautiful coloured sapphires.

The appreciation of colour shades may vary from region to region depending on taste and cultural symbolic aspects.

To appreciate, let's learn to look at the details with a magnifying glass!

 

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