Brown diamonds like their yellow counterparts, most often receive their color from exposure to nitrogen during the long formation process.
The GIA’s standard D-Z color scale rates the degree of yellow or brown tint in a “white” diamond, but brown diamonds also have their own color scale named after the famed Argyle Mine in Western Australia – the source of the majority of the world’s brown diamonds.
Argyle grading assigns brown diamonds both a description and a letter and numerical grade: Very Light Brown is called Light Champagne and graded C1-C2; Medium Brown is termed “Medium Champagne” and graded C3-C4; Brown is called “Dark Champagne” and graded C5-C6; while Dark Brown is known as “Fancy Cognac” and graded C7.
In 1967, a brown diamond weighing 601 carats was discovered in the Kingdom of Lesotho. Further exploration of the kimberlite that yielded the Lesotho Brown, as the stone came to be known, led to the development of the Letseng Mine, owned by Gem Diamonds.
Harry Winston bought the Lesotho Brown in 1968 and cut it into 18 diamonds with a total weight of 242.50 carats. The largest is the Lesotho I, a 71.73-carat Flawless emerald-cut diamond. Interestingly, the Lesotho I is a pale pink color. Since the stone contains no nitrogen impurities, it is believed that the rough diamond must have had a pinkish-brown color produced through plastic deformation (changes in shape resulting from a defect or irregularity in the diamond’s crystal structure.)
The largest brown diamond known today is the Golden Jubilee, a 545.67-carat cushion-cut Fancy Yellow Brown stone that is more than four times bigger than the next-largest brown diamond, the Star of the South – a 128.48-carat cushion-cut diamond with a color grade of Fancy Light Pinkish Brown.