About karats

ABOUT CARATS
It is important not to confuse the purity carat (symbol kt) with the weight carat (symbol ct).

Carat is the name of two different units:

• CARAT WEIGHT – a unit of weight used for gemstones and pearls
• CARAT PURITY – a unit of fineness used for gold alloys

CARAT (ct) – WEIGHT
Carat “ct”

• a unit of weight used in jewellery for gemstones
• not part of the SI system (the internationally agreed system of units and physical quantities – length, mass, time, electric current, thermodynamic temperature, luminous intensity, amount of substance)
• one metric carat equals exactly 200 mg (0.2 g) – 10 g therefore equals 50 carats
• (ct, Ct, CT – all variants are correct)
  This unit was historically derived from the weight of the carob seed, used in the Middle Ages in Arabia and Persia to determine the price of gemstones. The reason was the availability and stability of the weight and size of these seeds.

CARAT (kt) – PURITY
Carat “kt”

• a dimensionless unit of fineness of gold alloys
• the fineness of pure gold is defined as 24 kt – Au 999/1000
• one carat corresponds to 1/24 of the mass fraction of gold
• the most common jewellery alloys today are 14 kt and 18 kt
• in addition to carats, thousandth ratios are also used – e.g. 14 kt gold = Au 585/1000 or 18 kt gold = 750/1000

When producing jewellery with a given fineness, other metals (alloys) are equally important, as they determine the final colour of the gold. 14 kt and 18 kt gold can be yellow, red, or white. Nowadays, goldsmiths usually do not alloy the gold themselves but buy or exchange it at jewellery wholesalers.