‘Golconda’ is a historic term that refers to the area around the city and fort of Golconda in south central India, in today's state of Hyderabad. The region remained the world's only source of diamonds, apart from a small supply in Borneo, until deposits were discovered in Brazil in 1725 (just about the time that the Golconda mines became depleted).
The name Golconda has now become a byword for the finest, rarest and purest of diamonds; the dream possession of every serious gem collector. It is indicative of the escalating and seemingly never-ending quest for ultimate one-of-a-kind gemstones or jewels. What distinguishes Golcondas from the vast majority of diamonds is their Type IIa designation, referring to gems that are devoid of nitrogen. The element, present in Type I diamonds, lends stones a slightly yellowish tinge. "Only 2 percent of all diamonds are found in this Type II category," said Thomas M. Moses, senior vice president of laboratory and research at the Gemological Institute of America, G.I.A. "Because they're so pure, they transmit UV and visible light that Type I diamonds block. They have a clear, limpid, transparent nature."
The biggest known Golconda diamond is the Regent, weighing more than 410 carats when discovered in the eighteenth century. The Regent was sold to the regent of France by the English prime minister (hence the name), and adorned the chapeau of Marie Antoinette and later the hilt of Napoleon's own sword. It is currently displayed at the Louvre.
The Darya-i-Nur (Sea of Light) is a rare blue-diamond that weighs 186 carats. It was owned by the last Great Mughal Emperor of Persia, Aurangzeb, until it was plundered from his heirs during the 'sack of Delhi' in 1739.
The most famous is probably the legendary Koh-i-Noor, a 186-carat stone which, when discovered, was spirited away to Great Britain and now forms a part of the Crown jewels.