The weight of a gemstone is measured in carats (ct). 1 carat equates to 0.2 grammes, or in other words, 1g = 5ct.

This means that certified gemstone weights are generally accurate to one hundredth of a gramme, or in rare cases, one thousandth of a gramme.

Karat or Carat?

Carats when discussing gemstones are not the same as karats when discussing gold. For gold, karat (kt) describes the purity of the gold alloy. Pure gold equates to 24 karats, 18 karat gold equates to 750 gold (75% pure gold), 14 karat gold only contains 58.5% pure gold (585 gold).

The origin of the carat unit allegedly goes back to the dried seeds of the carob tree, which were used as balance weights for measurement. In reality, the carat as a unit differed from country to country until the year 1907, when the metric carat (carat métrique) was introduced as a standard international unit. Since then, a 1 carat stone has 0.2 grammes.

The metric carat (carat métrique), with its value of 0.2 grammes, was not introduced as an international unit of measurement until 1907.

How big is a 1 carat stone?

Generally speaking, the “size” of a gemstone is actually its weight. This is due to the fact that the actual dimensions of a 1 carat stone depend on both the density of the mineral and the type of cut employed.

A 1 carat brilliant cut diamond has a diameter of 6.5mm, while a round cut ruby of 1 carat is only 5.35mm in diameter. The reason for this is the difference in specific density of the two gemstones.

the world’s largest gemstones

The largest rough diamond ever found, internally flawless into the bargain, was discovered in South Africa in 1905 and named “Cullinan” after the owner of the mine, Thomas Cullinan. The diamond weighed 3,106.75ct and was divided into many pieces. The nine largest stones, (Cullinan I – Cullinan IX) form part of the British Crown Jewels.

The largest cut ruby in the world is the Liberty Bell Ruby. It weighs 8,500ct (1.7kg) and to celebrate the United States Bicentennial it was cut in the form of the Liberty Bell. This stone is, however, opaque.

The largest sapphire was found as recently as 2015 in Sri Lanka. The “Star of Adam” weighs 1,404.49ct (just under 281g).

The largest cut emerald in the world, considered “a gift from God”, is named “Teodora” and weighs 57,500ct – that is 11.5kg. However, there is doubt among experts as to its authenticity.

Cullinan 9 Major Diamonds
The nine large Cullinan diamonds (photo: public domain)

Price per carat: how much does 1 carat cost?

In contrast to precious metals, it is not possible to define a standard price per carat for gemstones.

In contrast to precious metals, it is not possible to define a standard price per carat for gemstones. Alongside its weight in carats, the following characteristics have a strong influence on the value of a gemstone:

  • Naturally coloured or treated
  • Colour and intensity (saturation)
  • Clarity
  • Source
  • Cut-type
  • Certificate
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Dr. Thomas Schröck
The Author:

Dr. Thomas Schröck

The founder and managing partner of THE NATURAL GEM has been active in international gemstone trading for 30 years. As a doctor of economics and a certified gemmologist in Switzerland, Germany and the USA, among other countries, he is one of Europe’s leading experts on naturally-coloured, untreated gemstones and investments in them.


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