Solitaire cushion shaped old mine diamond ring, French, circa 1950.

In reference to gemstones, a unit of weight, abbreviated 'ct'. 1 carat is equal to 0.2 grams. 

In reference to gold, a unit of purity or fineness of gold and gold alloy, expressed as a number out of 24 parts by weight, e.g. '24 carat' signifies pure gold, '18 carat' 18/24th gold in the alloy, et cetera.  Also abbreviated as 'ct'.

Originally derived from the carob seed, called quirat in Arabic, a seed of naturally uniform weight.

The term 'old mine' refers to gemstones deriving from historical, often ancient, mining sources.

With regards to diamonds, for example, this would indicate that the stones were mined in India or Brazil, the two primarily mining locales prior to the discovery of diamonds in South Africa in the late nineteenth century. India in particular is renowned for diamonds mined in the central area of the subcontinent surrounding the city of Golconda, and are known for their particularly high quality in terms of size, colour and clarity.  The Hope diamond and the Koh-i-Noor, perhaps the two most famous diamonds of all time, are Golconda diamonds. To add to the rarity of such stones, these mines have been largely exhausted.

In the case of emeralds, stones classified as old mine would be from Egypt, Austria, Gandahara (the ancient central Asian kingdom) or, primarily, Colombia.  The highest quality old mine emerald material derives from this last source, which had been mined by pre-Colombian tribes for approximately one thousand years.  It was not until the Spanish presence in the New World in the early sixteenth century, however, that these stones graced the royal courts of Europe, the Middle East, and, primarily, India.

Diamonds have been prized for their unique physical attributes for millennia.  Formed of crystallized carbon, they are the hardest substance on earth.  It is unsurprising then that diamonds have long been the symbol of strength, invincibility and eternal love. 

The first significant source of diamonds was India, more specifically a region known as Golconda.  These diamonds are particularly prized for their lack of impurities, resulting in colourless diamonds of supreme clarity and brilliance. Many of the world's most famous were found in the Golconda mines, including the Hope diamond and the Koh-i-noor. 

From India diamonds were carried along the Silk Routes of Central Asia, through Turkey and thence on to Europe.  It was during this time that Venice became a major diamond trading centre, with Bruges and, later, Antwerp at the northern end of the route.  India remained the primary source of diamonds until the eighteenth century, by which time the mines there had been largely depleted.  Rather fortuitously, around the same time diamonds were discovered in Brazil.  This source, however, was short-lived, and ran out in the mid-nineteenth century. 

But yet again, a new source immerged to replace it, this time one of much more significant supply.  In 1866 a child in South Africa found an unusual looking stone, which turned out to be a twenty one carat rough diamond, now known as the ‘Eureka' diamond.  Shortly thereafter, in 1869, the discovery of an 83.5 carat diamond-the ‘Star of South Africa'-confirmed the significance of the deposits.  

The now famous DeBeers Company, founded by Englishman Cecil Rhodes, controlled all of the diamond deposits in South Africa from the time of its establishment in 1888.  And so it came to pass that London became the world's rough diamond trading centre, as all stones passed through his London offices, while cutting carried on in Antwerp, and later Tel Aviv and New York.  Today South Africa remains a major source of world diamonds, joined in the twentieth century by Canada, Australia and Russia, which helped to break down the DeBeers monopoly.

Though most people think of diamonds as colourless, diamonds can form in most any colour of the rainbow, including black.