About Jewellery » Karats or Carats?
Or is it karat? Well they are derived from the same word, the word for the fruit of the carob tree, which in Italian is carato; in Arabic qirat; in Greek, keration. In ancient times the seeds of the fruit were used for weighing gems, which is why we also see the word 'carat' to refer to the weight of diamonds and other gems.2 The pure gold Byzantine coin called the solidus weighed 24 karats and thus the term '24 karat' came to indicate pure gold. Fineness is another term used to indicate purity: fine gold is pure gold; karat gold is gold that has been alloyed.
So, if 24k is 100 percent gold, 18k is 75 percent gold (18 being three-quarters of 24), and 14k is 58.3 percent gold. Besides the 'k' or 'kt' mark on jewellery, you may also see numbers indicating parts per thousand. Thus, 24k gold may also be stamped '1000', 18k stamped as '750', and 14k as '585' (oddly, not 583). There are legal standards for what can be called gold. In North America, to be called 'gold', the item must have a minimum of 10k. In Italy and France standards are much higher where pieces must be a minimum of 18k to be called 'gold'. The term karat is only used for gold, not for platinum or silver.
Alloying precious metals changes their properties in order to make the metals more durable and workable, to alter appearances (e.g. colour), or to make the metal more affordable.
While there are many colours of gold jewellery available, gold as a natural element is yellow. The colours are manufactured by alloying (mixing) the gold with a specific combination of other metals. For example, white 18k gold will have 75% gold, 25% platinum or palladium or it will be 75% gold, 10% palladium, 10% nickel and 5% zinc. Red, rose and pink 18k gold will include different levels of copper within the alloy mix, depending on how deep the red is desired. Yellow gold that has been alloyed (such as to make it 18k or 14k gold) but remains yellow is also mixed with alloys, including small amounts of silver, copper and zinc. These are the colours we are familiar with, but there's also green, blue, purple, brown and black gold! Platinum is also alloyed, but it is alloyed with metals of its same family, such as ruthenium, iridium, palladium. Platinum alloys that are 950 parts per 1000 platinum and 50 parts of other platinum family metals will still be stamped as 'platinum' (or with just the letters 'Plat').
Silver too is alloyed because in its pure form it is too soft. In North America, to be stamped as 'silver' the metal must be 92.5 percent pure, which is why you'll usually see the stamp '925' on sterling jewellery; the other 7.5 percent is copper.3
2 In North America, we spell the karat for metal with a 'k', with the abbreviation 'k' or 'kt', and the carat for stones with a 'c'. In Britain and other countries the word is spelled with a 'c' for both for gold purity and gem weight.
3 Careful, 'nickel silver' doesn't contain any silver at all but is a combination of the base metals copper, nickel and zinc - kind of like 'fools gold'.