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Blog - Grand Auctions

High Pressure High Temperature Diamonds


High Pressure High Temperature, or HPHT, describes a form of treatment that can be applied to diamonds in order to improve their colour. In this article we take a brief look at the process and explain what differentiates them from the real thing.

10 diamonds in 30 minutes

As the name suggests, diamonds are exposed to high pressures of up to 60 kilobars and high temperatures of about 2000°c for a short period of time. Ten diamonds can be processed in around half an hour. The process was first used in the mid-1980s but didn’t really take off until the late-1990s.

Wide range of colours

At first only brown to grey coloured diamonds of high clarity were treated, with the resulting stones having an improved colour. Today a wider range of diamond types and colours can be treated. Yellow or brown diamonds can be transformed into strong yellow-green or fancy yellow stones, yellow to brownish stones can be turned yellow, brown stones can become colourless and, rarely, pink and brown-grey diamonds turn blue.

How to spot them

To the untrained eye these diamonds are difficult, if not impossible, to differentiate from untreated stones. Pitting and burn marks may be present on surfaces and fractures and tension marks may appear around inclusions within the stone. Distinctive grain patterns can be identified within the diamond, sometimes with the aid of UV light. Sometimes the stones appear slightly hazy or have a strong, characteristic glow under UV light. Other times a spectroscope can be used to test how the spectrum of visible light is modified by the diamond as it passes through it. Most of these methods will almost certainly require the skill of a gemmologist and probably within a laboratory setting at that.

Effect on value

Within the world of gem buying, only natural, untreated stones command the top prices. Despite the fact that HPHT treatment is a permanent process, diamonds altered in this way are not worth anywhere near as much as their untouched counterparts. Discounts ranging anywhere from 20% - 80% are sometimes quoted. So much will obviously depend on the quality of the finished product, just as it does when considering untreated stones. But if you are looking for a large, colourful diamond at a fraction of the price of an untreated stone, HPHT might just be worth considering.

If you have any diamonds that you would like to sell, please contact Simon Rufus, gemmologist at Grand Auctions, Folkestone, Kent for further details.

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