Brown diamonds, the most common fancy coloured diamonds, owe their colour to so-called laminar deformation. This is caused by slippage or gliding of crystal planes parallel to the natural cleavage plane in a diamond, also known as the octahedral plane. Laminar deformation impacts selective absorption along the entire colour spectrum with its effect being greatest in the green and blue parts. As a result, returning light recombines as visible brown to the viewer. This type of deformation also causes a lot of internal (cross) strain and is often visible as surface reaching ‘grain-lines’ which may appear as ‘stitches’ on a diamond crystal’s surface.
Generally, the more deformed a crystal is the darker its colour. And the more saturated the colour, the greater the chance of the hue being modified. A further more practical implication is that brown stones, particularly those from the Argyle mine, can be challenging to polish. It is also no coincidence brown and pink diamonds commonly occur together. Excepting Type IIb, brown diamonds can be of any type.
Brown like yellow and grey is graded on the normal GIA colour scale. Thus only saturations beyond Z are considered ‘Fancy’. Diamonds with the faintest touch of brown are known in the trade as TTLB (top light brown) and TLB (top light brown ). They often face up as white or near-white. K to M is considered faint brown, N through R as very light brown and S to Z as light brown. Approximately 30% of all brown diamonds strictly fall into the category Fancy Brown. The darkest are visually nearly black. In order to promote brown diamonds further, Rio Tinto, operator of the Argyle mine, introduced the alternative ‘C’ – Chart. It classifies brown diamonds according to increasing saturation from C1 (Light Brown) through to C8 (Deep Brown).
For fancy brown diamonds to be commercially desirable they must be bright and hold a pleasant, preferably warm colour. Beautiful browns are fairly uncommon with some hues being even rarer than fine colourless diamonds. Such stones may represent great bargains to the savvy buyer. Secondary colours including yellow or orange or the even rarer pink or elusive red are highly desired in brown diamonds.
The most popular brown diamond varieties include Cognac, a dark orangy Brown and Champagne which faces up as a light (greyish) yellowish Brown to Yellow-Brown. Chocolate features deep pure Brown, whereas Caramel (from Mbuyi Mayi) stones are a distinct Orange-Brown. Amber coloured diamonds which are yellow-orange brown are also popular. This particular hue is caused by a colour-centre of the same name showing up as a spectral line at 480Nm.
Many more brown hues exist containing minor green, grey, olive or even rare and valued purple. However few of these are beautiful since the more secondary hues brown stones contain the less attractive they become. These modifiers may be expressed via an undesirable multicolour effect, an interplay-of-colour which unlike in gemstones is caused by both colour zoning and the visual angle of incidence. Rarely also bluish to greenish brown diamonds are seen in which linear strain causes a rather unattractive ‘oily’ blue to green fluorescence.
Brown diamonds are abundant in Russia, the Republic of South Africa, Australia, the Central African Republic, Guinea as well as Canada and Brazil. Just about any mine conceivable will produce some brown diamonds.
Throughout the last few decades pure Type IIa browns, which are affected mostly by plastic deformation and rarely contain insignificant amounts of other colouring agents, have been selected for the so-called High-Pressure-High-Temperature Treatment. This procedure tries to eliminate the brown and convert the diamonds to much whiter colour grade. Improvements of up to 5 colour grades are common. HPHT treatment tries to achieve this by ‘relaxing’ the crystals and strengthening their crumpled structure. HPHT diamonds must be disclosed and accompanied by specific HPHT grading reports but are otherwise graded like colourless diamonds.