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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 4th to 10th

centuries approx equal to 189/187mg

• It is thought that the carob seed was chosen

because it was more consistent in weight than

• ther commonly available alternatives (grain)

Gemstones – a Unit of Weight

The “Solidus” A Roman/Byzantine Gold Coin

Gemstones – a Unit of Weight

• From the early 4th through the 10th 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. Ière 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

• ther 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

• f 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