The April gemstone is the diamond, the epitome  of a precious gemstone. Diamonds are transparent, with an adamantine lustre and showing very high refraction. They are the hardest gemstone but can cleave if knocked. Diamonds come in lots of colours – pink, green, blue, yellow, black, brown – but traditionally it is the colourless diamond that is commonly used for jewellery.

For over 2300 years, the main source of diamonds was India until the gemstone discoveries in Brazil in 1725. Until the beginning of the 1870s, diamonds were found in rivers and in gravel but the discovery of volcanic pipes of diamond bearing rock in South Africa changed the way diamonds were extracted.

The most significant changes made to the way diamonds have been used in jewellery is the way they have been cut. Originally, diamonds were roughly smoothed, with no further work done to them. Gradually, as equipment and techniques improved and with the use of diamond dust as a polishing tool, more polishing and faceting was done, starting from simple one or two facets to more complex cutting. The main cuts of diamonds used in the 18th and 19th centuries were rose cuts and old mine cuts, cut and faceted by hand.

The rose cut is named after a rose bud as the facets on the top of the diamond are supposed to resemble a rose bud, just beginning to open. A rose cut diamond is round, has a flat bottom and has a domed top. It may have between six and 24 facets.

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Victorian bar brooch with rose cut diamond-set coiled snake with ruby eyes (in Navette on Ruby Lane)

The old mine cut diamond (also known as old cushion cut diamond)  is a squarish shape, with a small table and a large culet. The name is believed to have arisen as a way to distinguish diamonds from the newly discovered mines in Brazil, and later, Africa, from the old mines of India. The name generally became applied to any squarish cut diamond with 58 facets.

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Georgian ring with old mine cut diamonds and rubies (in Navette on Ruby Lane)

Mechanisation by the 1880s was to lead to more control over cutting and facetting diamonds and to the development of the Old European cut, the Asscher cut and finally the Brilliant cut.

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Georgian brooch with old cut diamonds and turquoise (in Navette in Ruby Lane)