Coloured diamonds are diamonds that have a noticeable colour compared to the typical ‘clear’ gemstones. Diamonds that are coloured red, pink, green, orange, red and blue are rare types of coloured diamonds. And then there are the commonly mined brown and yellow diamonds. These kinds of diamonds are sought after their fantastic colours, especially when polished by a master cutter. Coloured diamonds are so rare that the probability of finding one is one in ten thousand.
What is the cause of colouration in diamonds?
A diamond is crystallised carbon and occurs as either type I or type II. It is produced when carbon atoms are compressed by intense pressure and heat over millions of years under the Earth’s crust. A perfect carbon crystal is colourless, and completely free of impurities, but due to events that may occur during the compression, a diamond will have distinct attributes and sometimes, colour. There are many explanations of why there are diamonds of various shades, but here are some of the most common reasons for each major colour variation.
These diamonds are the rarest type in the pink diamond range as well as the most expensive. As a diamond is formed, extreme pressure can cause the layers of carbon atoms shift in planes or sheets. Some colouring may occur due to the interaction of light as it passes through the diamond’s displaced glide planes. The minuscule displacements causes white light to transmit or reflect in the red wavelength and this distortion makes the diamond appear red. The Argyle mine in Australia has produced a small quantity of quality Type Ia red diamonds, and a few Type II reds from other locations have found themselves centre stage.
Purple and Violet Diamonds
After red diamonds, purple diamonds rank as the second rarest type of coloured diamonds. The purple colourisation is theorized to have H3 or triatomic hydrogen as an impurity. When carbon atoms are replaced by hydrogen, light passing through the stone is transmitted as a purple tint. Purple diamonds make up 1% of the diamonds mined in the Mir mine in Siberia.
Blue diamonds are caused by the presence of boron atoms in the carbon crystal. The molecule causes the absorption of red light and thus transmit blue light. Blue diamonds have been found in the Cullinan mine of Pretoria, South Africa. The Australian Argyle mine produces an even smaller amount of Type Ia blue diamonds.
The same distortion of carbon atom layers seen in red diamonds is apparent in diamonds with a pinkish hue. The displacement happens when a diamond encounters extreme pressure as it travels from the mantle to the crust. However, the displacement planes are not enough to entirely transmit an intense red light; presenting as a pink hue instead. Most Type 1a pink diamonds are from the Argyle mine in Australia, and Type II pink diamonds have been mined from mines located in Tanzania, India, and Brazil.
Yellow and Brown Diamonds
During millions of years of intense pressure, the carbon material may become contaminated with nitrogen. As the carbon crystallises, the nitrogen molecules are also trapped and becomes an impurity. The yellow hue is caused by nitrogen as it refracts yellow light. Among all of the coloured diamonds, yellow diamonds are the most common type. In fact most ‘colourless’ diamonds, as Type I’s, contain a small amount of nitrogen, and display yellow to brownish tints. Yellow diamonds have been found in South Africa and Russia.
This type of diamond is one of the rarest next to the purple diamonds. The effect of radiation causes the greenish hue as the diamond travels to the crust. In some cases, nickel, nitrogen and even hydrogen impurities may cause green colourisation. However, the green tint is mostly on the diamond’s surface; thus, its colour is lost after thorough polishing.
This type of diamond gets its hue from impurities like graphite, pyrite and hematite. The presence of these minerals makes black diamonds prone to fracture. Usually, black diamonds are opaque, though some have clear crystal mingled with black patches. Black diamonds are typically used for industrial purposes such as for drill bits and knife edges.
Where are coloured diamonds mined?
Diamond mines are surprisingly rare. The mines that regularly extract diamonds from the Earth are:
Golconda Region Mines, India
From antiquity, India has long been a treasure trove of diamonds. It is because of the abundant mineral deposit in the Golconda region, which is now in Andhra Pradesh. It is from these mines that a lot of the famous diamonds came. The world-famous Hope Blue diamond, which many people say is cursed, came from the Kollur Mine, one of this region of mines.
The Argyle Mine, Australia
This Australian mine is a significant source of the world’s supply of pink, blue-violet and red diamonds. Coloured diamonds mined here are of such a high quality that some command a price of $1 million per carat. Aside from this is the abundance of brown diamonds, which today became a signature gem of Le Vian as their “chocolate diamonds“. With the closure of the mine occurring in 2020, Argyle stood out as the only mine to consistently produce pink diamonds.
The Alrosa mines of Russia
The Alrosa is a mining conglomerate in Russia. It is a reliable source of red, blue, green and violet diamonds. The expansive project of Alrosa in Russia is moving them into the position of world’s most reliable producer of coloured diamonds.
Other sources of diamonds are listed below:
- Cullinan Mine in South Africa
- Jwaneng, Botswana, Africa
- Diavik, Canada
- Debmarine, Namibia
Artificial Diamond Production
As technology advances, so has the production of artificial diamonds. Mimicking the pressures involved in the natural production of diamonds, laboratories can create fully-synthetic diamonds. However, because it is synthetic, it has lesser value despite looking similar. The development of artificial diamond production allowed a large number of people to enjoy diamonds in their jewellery. Aside from diamond production, the various hues and shades of natural coloured diamonds can be artificially replicated as well. However, as the proliferation of artificial diamonds are flooding the market, expert appraisers are always on the lookout when separating natural diamonds from the rest.
How is the price of coloured diamonds determined?
Determination of the value of coloured diamonds is done by independent gemstone appraisers, skilled in techniques for identifying natural and lab-produced colourisation. Factors for estimating any value are the following:
- Determining first if the material is an authentic diamond or lab-created
- Determine if the hue is natural or artificially achieved
- Determine if the diamond has undergone any laboratory treatments
- Lastly, to determine the colour grade of the gem.
Colour grading the diamond is an essential process in determining its absolute value. The colour-grading system involves determining if the hue is natural, and compared against a master set of authentic specimens. The cost of a diamond depends on this process, which is why experts conduct colour grading in the field.