Gemstones – a Unit of Weight
Gemstones – a Unit of Weight • The historical unit of weight for gemstones has been the “Carat” - the weight of a single seed from the seedpod of the carob tree (Ceratonia Siliqua) hence latin - siliqua (carat) • The Roman/Byzantine carat 4 th to 10 th centuries approx equal to 189/187mg • It is thought that the carob seed was chosen because it was more consistent in weight than other commonly available alternatives (grain)
Gemstones – a Unit of Weight The “Solidus” A Roman/Byzantine Gold Coin
Gemstones – a Unit of Weight • From the early 4 th through the 10 th century, a gold coin, the solidus, played a preeminent role in Roman and Byzantine coinage. It weighed 24 siliqua , and from this the carat weighed 24 siliqua , and from this the carat became identified with the fraction 1/24. Reference E. Babelon. Traité des monnaies grecgues et romaines. I ère Partie. Théorie et doctrine. Paris: Ernest Leroux, 1901 .
Carat vs. Karat • Carat(s) is the unit of weight - one of the 4Cs • Karat is a proportional measure of purity in the metal Gold. Pure gold is too soft for any usage involving wear. It must be alloyed with usage involving wear. It must be alloyed with other metals • 24 Karat is pure gold - 24 parts gold to 0 parts alloys/impurities (24/24ths to 0/24ths) • 18 Karat gold is 18 parts gold to 6 parts alloys/impurities (18/24ths to 6/24ths)
Gemstones – a Unit of Weight • Early Syrian/Arabic carat approx 212mg • Caliph Abd al-Malik (646-705) instituted a mint that produced a uniform set of aniconic currency. In doing so he redefined the carat as currency. In doing so he redefined the carat as 1/20 Mithkals • The mithkal is a unit of weight in Iran for measuring gold. It is a little under 5g making an Arabic carat a little under 250mg
Gemstones – a Unit of Weight • By the nineteenth century the carat was still the unit of weight for gemstones but the weight of the carat varied from country to country • In Italy, four different gem-center cities each • In Italy, four different gem-center cities each had a different value for the carat Bologna carat, 188.6 Florence carat, 196.5 Turin carat, 213.5 Venice carat, 207.1
Gemstones – a Unit of Weight • In 1907, the General Conference on Weights and Measures adopted a standard for world trade tied to the gram • The gram (or kilogram) is the international The gram (or kilogram) is the international standard of weight established by the Meter Convention • Known as the metric carat, it weighs 200 milligrams – i.e. 5 carats equals one gram • The USA adopted this standard in 1913
Gemstones – a Unit of Weight • The metric carat is divided into points • 1 carat = 100 points • In the trade, diamonds or any gemstone, weighing less than 18 points is referred to as weighing less than 18 points is referred to as melee • The Indian diamond trade has become huge (55% to 65% by volume) because of their expertise in cutting tiny, fully faceted (57/58) diamonds and low quality diamond melee
Gemstones – a Unit of Weight • Diamonds are weighed to 3 decimal places and quoted to 2 decimal places • The FTC allows diamond weight to be quoted in trade rounded to 2 places of decimals. • .005 and higher rounds up - .004 and lower .005 and higher rounds up - .004 and lower rounds down • 0.734cts rounds down to 0.73 • 0.735cts rounds up to 0.74 • 2.00cts quoted - actual weight 1.995 to 2.004
Gemstones – a Unit of Weight • the World Federation of Diamond Bourses requires a more consumer friendly rounding standard, which GIA Labs has adopted • .009 rounds up • .009 rounds up • .008 and lower rounds down • 2.00cts quoted - actual weight 1.999 to 2.008
Gemstones – a Unit of Weight • The vast majority of cut diamonds are less than 1ct in weight • One carat is the first break point in value • Rough diamond crystals over 10.8cts are sold Rough diamond crystals over 10.8cts are sold by de Beers in special sales not part of the regular 10 sites per year. This size of crystal will cut about a 5ct finished stone • Diamonds over 5cts are rare
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