Natural Diamonds
— Earth's gift of exquisite beauty
As well as being valued for their captivating beauty and hardness, diamonds have even been thought to possess magical properties – in early India, just to gaze upon a diamond was considered strengthening.
What is a natural diamond?
Thought to have been formed between one and three billion years ago, diamonds are one of the oldest substances known to man.
Diamonds were formed deep within the Earth’s mantle between 140 to 190 kilometres below surface, where the high temperature and pressure conditions resulted in the crystallisation of carbon into diamond. The diamonds were then forced through the Earth’s upper mantle towards surface, too fast for their crystalline structure to degrade to graphite, during rare volcanic eruptions of molten rock. As the volcanoes cooled, the magma hardened into carrot shaped ‘pipes’, known as kimberlites.
Not all diamonds would have survived the ascent, meaning that each and every natural gem diamond is a true miracle of nature.
White natural diamonds
Pure white diamonds are in fact very rare, with yellowish or brown tints being present in the majority of ‘white diamonds’, albeit so faint in many cases that it is undetectable by the amateur observer.
The hint of yellow that can be noted in many diamonds is caused by the element nitrogen, present within the carbon structure. Diamonds containing nitrogen are classed as Type I, whereas those containing aggregated nitrogen atoms are classified as Type Ia (the most common category of diamonds). Diamonds without any detectable nitrogen atoms are exceptionally rare and are known as Type II, with a sub-division of Type IIa, being the purest.
At Petra
Cullinan Mine is renowned for producing some of these most precious gems.
Coloured natural diamonds
Nature produces diamonds in every imaginable colour, with over 300 shades and hues having been identified to date. However, it is estimated that only one in 10,000 diamonds has a ‘fancy colour’. The premium prices commanded by ‘fancy coloured’ diamonds are dependent on their tone, saturation and distribution of colour.
A diamond usually comprises more than 99.7% carbon, but the remaining 0.3 per cent constitutes traces of impurities of other elements during its formation. A diamond’s colour is generally determined by such chemical impurities, but it can also be caused by a distortion of the crystal lattice.
At Petra
Petra’s mines produce some of the world’s most precious coloured diamonds, with record prices achieved for Cullinan Mine blue diamonds and Williamson pinks.
Blue Diamonds
Blue diamonds are one of nature’s rarest treasures and Petra’s Cullinan Mine in South Africa is the world’s most important source of them. The natural blue colour is derived from the chemical element Boron, which is trapped in the crystal carbon structure during their formation. A diamond can be blue with less than one boron atom per million of carbon atoms. Petra has recovered and sold a number of important blues since taking over the Cullinan Mine, including a 39.34 carat blue diamond sold for US$40.2 million (cUS$1 million per carat) in July 2021 – believed to be the highest price per carat on record for a rough diamond.
Pink Diamonds
Pink diamonds are one of the most highly coveted of all the fancy colours and have only been found in a handful of mines throughout the world. The pink colour is thought to be brought about by an alteration to the diamonds’ molecular structure, possibly caused by a seismic shock at the time of its formation. Petra’s Williamson mine in Tanzania is now considered the world’s most important source of pink diamonds and is renowned for its ‘bubblegum’ pinks, including the 23 carat Williamson Pink, which is considered the finest pink diamond ever discovered and the Williamson Pink Star which set a world record for the highest price per carat (c.$5m) for a diamond sold at auction in 2022.
Yellow Diamonds
Yellow diamonds have long been valued and prized but they were only discovered sporadically until the late 1860s in the Cape Province in South Africa, where their relative abundance prompted the name ‘cape stones’ – a term that continues in usage today. The yellow hue is due to the presence of nitrogen within the crystal structure, where a carbon atom is replaced by nitrogen. Petra produces yellow diamonds from all of its operations.
Champagne/Cognac Diamonds
Champagne diamonds are light brown in colour, whereas cognac diamonds have a more orange-brown hue. Champagne/cognac diamonds have a vast range of tones, ranging from very light to very dark, often with modifying colours, such as hints of orange, red or yellow. The largest faceted diamond on record, the 545 carat Golden Jubilee, is golden brown and was polished from a huge stone recovered from the Cullinan Mine in 1986. Petra’s Cullinan Mine produces a regular supply of champagne diamonds.
Orange Diamonds
Pure orange diamonds are incredibly rare and therefore highly coveted by collectors. The orange tone is usually caused either by the presence of nitrogen or by an absorption band centred at approximately 480 nm.
Green Diamonds
Natural green diamonds are exceptionally rare, second only to red diamonds in their rarity. They are believed to owe their colour to millions of years of exposure to radioactivity, emitted by uranium compounds. The vast majority of green diamonds are only green in their rough form, with stains or coatings on the surface of the stone, which tend to be lost in the cutting process – these are known as ‘faceted’.
Red Diamonds
Although all natural fancy coloured diamonds are extremely rare, none are more so than the red diamond. Only 20 to 30 true red diamonds are known to exist and most are less than half a carat in size. Unlike other fancy colour diamonds that derive their colour from impurities, red diamonds are made purely from carbon, just like colourless diamonds. Due to their incredible rarity, red diamonds hardly ever come to market, but in November 2014 a heart-shaped 2.09 carat red diamond ring by Moussaieff sold for US$5.09 million.
