What does carat mean?

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Carat or Karat

In the UK and other countries, carat applies to both gold and gemstones. In the US, carat applies only to gemstones - when applied to gold it is spelled karat .
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Carat meaning

Carat, when applied to gemstone, is a measurement or unit of weight.

In measuring gemstones, one diamond carat weight is one-fifth of a gram or 200 milligrams. Carat diamond weight can be divided into smaller units of points. One point being a hundredth of a carat. A 50 point stone is, therefore, half carat and would weigh 100 milligrams.

Carat when referring to gemstones is internationally abbreviated to CT or ct. Carat measures the weight of a diamond and not the diamond size.

Because pure gold is extremely malleable, manufacturers such as jewellers generally mix or alloy precious metals with harder metals. Without knowing exactly how much pure precious metal an item contains it is difficult to arrive at its true value. Carat, when applied to gold, measures the proportion of gold to other metals in an alloy. It shows the purity of gold or gold alloys and enables better evaluations to be made.

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Gold carat measures the proportion of pure gold to other metals as parts of 24. Pure gold is 24-carat gold. Nine carat is nine parts gold: the remaining 15 parts of 24 being composed of other metals.

Gold carat is abbreviated to CT or ct and in the US Karat is abbreviated to k.
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Origin of 'carat'

In ancient times, gold and precious metals were weighed against carob seeds. The term 'carat' is derived from the Greek kerátion / κεράτιον , meaning 'carob seed'. After 1570 there is evidence that this weight was also began to be used for pearls and gemstones.

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Carob seeds, however, are not all identical and vary in weight. Because of this, before 1907 there were divergences from across national borders. In that year, however, at the fourth sexennial General Conference of the Metric Convention, an international carat standard was agreed. This set the modern carat weight at its current 200 milligrams.
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Why is gold carat twenty-four parts?

The ancient Roman gold 'Solidus' coin is thought to have weighed 24 carob seeds. This association between the carob seed or carat and 24 parts is thought to be the reason for a gold carat being in the ratio of 24 parts.
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When is gold not gold

Pure gold is too soft for most practical uses. It follows, then, that when talking about gold items in truth we are referring to gold alloys. Certain alloys contain so little gold that they can no longer be called gold.

This minimum gold content varies by country. In the US, 10 carat is the legal minimum and 14 carat is commonly used. In France, the UK, Austria, Portugal and Ireland, 9 carat is the minimum that can be called gold. In Denmark and Greece it is even lower at 8 carat.
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Fineness

As well as carat, another way of expressing the proportion of gold in an alloy is Fineness. Fineness is an alternative decimal based system and shows purity in parts per thousand. Fineness of 999.9 is pure gold. It is equivalent to 24 carat.

Other common UK fineness standards are:

Carat	Fineness
22	916
18	750
14	585
8	375

With the exception of coins and pure gold bars, in the UK any manufactured gold products weighing over 1g must be hallmarked with the items fineness.