By Renée Newman

Last October, the Williamson Pink Star set the record for the highest price-per-carat ever paid for a gemstone at auction—US$5.17 million per carat. The 11.15-carat Fancy Vivid pink diamond was sold at Sotheby’s Hong Kong auction for US$57.7 million, marking the second highest amount ever paid for a gem at auction.

Natural pink diamonds are exceedingly rare. What’s more, they are certain to become even more rare now that Australia’s Argyle Mine, which produced the bulk of them, has closed. However, thanks to modern technology, your customers can still own one of these precious pinks at an affordable price, and also enjoy the brilliance, sparkle, and durability diamonds offer.

Due to the rarity and popularity of coloured diamonds, scientists were driven to experiment with ways of changing the colour of light-yellow and brown natural-mined diamonds. Over a period of many years, they discovered ways to create a wide variety of colour-enhanced diamonds (Figure 1).

Colour enhancement methods

Irradiation

Irradiation is the most common process for colour enhancing diamonds. It is done in a government-certified laboratory, and the same machine is used to irradiate many other things, such as meat and surgical instruments. Laboratory irradiation involves two steps: electron irradiation (with low energy electron bombardment) and annealment (i.e. heating). However, it is usually identified as simply “irradiation.”

The process of electron irradiation gives diamonds a greenish blue or blue colour. In order to produce a multitude of hues, irradiation must be followed by annealment in an oxygen-free environment to high temperatures of 450⁰C (842⁰F) and upward. Figure 2 shows the “before and after” of a light brown diamond that was transformed into an orange-red diamond with irradiation followed by annealment.

Low pressure, high temperature (LPHT)

Most black diamonds today are created by heating heavily included diamonds at low pressure to temperatures above 1300⁰C (2372⁰F) to graphitize the fractures and turn them black. Unlike irradiated black diamonds, which usually have a bluish to greenish tint in transmitted light, heated diamonds are pure black. In addition to this favourable colour, heated black diamonds cost less and are valued based on surface impurities. Figure 3 shows an example of heated black diamond beads.

High pressure, high temperature (HPHT)

The most expensive colour-enhancement method, HPHT treatment reproduces conditions that create diamonds within the earth. Only diamonds of very high clarity can be used for this process, as inclusions can worsen or explode during treatment. Sophisticated cubic press machines expose the diamonds to heightened heat and pressure, resulting in a change in colour from brown to colourless, yellow, and green. Figure 4 depicts a light-brown diamond, which, through use of HPHT process, was changed to canary yellow.

In 1999, General Electric and Lazare Kaplan International announced they were able to turn Type IIa light-brown and light-yellow diamonds into D- to H-colour diamonds with HPHT processing. If there is an asterisk next to the colour grade on a diamond grading report, it is advised to check the comment section to see if the diamond was treated.

HPHT-processed mined diamonds are sometimes irradiated to turn them a different colour, such as blue or pink. Afterwards, they are sold as irradiated diamonds. Before irradiation, the VVS blue diamond in Figure 5b was the F-colour HPHT-treated mined diamond in Figure 5a.

Detecting colour-enhanced diamonds  

Irradiated diamonds can sometimes be detected with a microscope. Two strong indications of irradiation are colour zoning, following facet patterns, and colour concentrations in the culet area, which sometimes resemble an umbrella (Figure 6).

Diamonds have characteristic colour spectrums, which can also help determine the origin of their colour. Irradiated pink and reddish diamonds often have a strong orange fluorescence, especially under long-wave ultraviolet (UV) light. Natural pink diamonds may also have a strong orange fluorescence, but they have different spectra. It is usually difficult for a jeweller to prove diamond colour is natural. As such, when buying a fancy colour diamond, it is always best to obtain a report from a major independent gemmological laboratory, stating the gem is of natural colour and origin. Additionally, always be sure to purchase diamonds and gemstones from a knowledgeable, ethical seller.

Relative pricing

Enhanced light-pink diamonds are the most expensive, as they are the rarest and most difficult to produce. Curiously, the price of these diamonds decreases as their colour approaches medium pink and red. One of the reasons for this is the original natural-colour brown or yellowish diamond must have a lighter and more expensive colour-grade than needed for darker enhanced colours.

New York-based diamond dealer Lotus Colors, Inc., lists the hierarchy of its enhanced colours from highest to lowest priced as follows:

• Light pink
• Medium pink
• Purplish pink and light blue
• Purple
• Yellow, red, medium greenish yellow
• Orange, green, medium blue
• Dark blue, dark green
• Black

Caring for colour-enhanced diamonds

Cleaning

Colour-enhanced diamonds can be cleaned just like other diamonds—using ultrasonics, steam, acid, or any jewellery cleaner. There is, however, one exception: Avoid using acids to clean heated black diamonds. Strong acids can give heated black diamonds a burnt whitish look.

Jewellery repair and resizing

The colour of irradiated diamonds is basically stable, but some stones can change colour if they come into contact with a jeweller’s torch When resizing irradiated diamond rings, jewellers should treat the diamond like an emerald, as both have a similar heat resistance threshold.

Indeed, it is necessary to protect the diamond from direct exposure to temperatures higher than 450⁰C (900⁰F). This can be achieved by covering the diamond with a saturated mixture of boric acid and denatured alcohol—or, alternatively, by using a special thick paste called “Heat Shield.” This paste can be removed by placing the ring in a pickle pot, which contains a special cleaning solution.

Settings

Irradiated coloured diamonds can be set in all setting styles; however, they cannot be wax-set, as this process involves high temperatures.

Benefits of color-enhanced diamonds 

Affordability

You can expect to pay hundreds of thousands of dollars for a one-carat, natural-colour red, blue, pink, or green diamond (if you can find one); however, coloured-enhanced mined diamonds of the same size can be purchased for much less than their natural-colour counterparts. Further, in most cases, they are less than the price of most colourless diamonds of the same size.

Availability

Finding intense blue, green, pink, orange, and purple natural-colour diamonds is extremely challenging (often impossible) because of their rarity. Matching them is even harder.

It is not difficult, however, to find matching colour-enhanced diamonds for the stackable rings in Figure 7 because the stones can be ordered consistently in a variety of shapes, colours, and sizes, including stones as small as one millimetre. In addition to being available in consistent shades, enhanced diamonds can be found in a wide variety of colours, as is demonstrated by the ring in Figure 8.

Resistant to chemicals

Most coloured stones are damaged by chemicals and many household cleaners. With the exception of heated black diamonds, colour-enhanced diamonds can resist all acids and other chemicals.

Resistant to scratching and abrasions

Diamond is the hardest natural substance, making it more resistant to scratching and abrasions than any other natural material. As a result, colour-enhanced diamonds are ideal for engagement rings and other pieces worn daily.

Supreme selection

If you have customers interested in a stone more unique than the traditional white diamond—but one that does not sacrifice brilliance—consider offering the option of colour-enhanced diamonds. They combine the brilliance, durability, and prestige of mined diamonds with the rich colours of other gemstones.

Renée Newman, BA, MA, GG, is a gemmologist, lecturer, and author of 14 gem and jewellery books. Her newest book, Diamonds: Their History, Sources, Qualities and Benefits, is published by Canadian publisher, Firefly Books. It is hardcover with jacket, 272 pages, and has 380 photos, plus maps, diagrams, glossary, and index. For more information about Newman and her books, visit www.Renee Newman.com.