Everyone knows that 22 karat gold is more expensive than 9 karat gold. But why? A karat (or carat) of gold is a measurement of how pure it is – the number of carats in a piece of gold indicates the percentage of pure gold in the piece. The chemical symbol for gold is Au.
Twenty-four karat gold is pure Au, and is seen only in bullion bars. Gold is an extremely soft metal which loses particles if used or handled a lot, so other metals have to be added to strengthen it before it can be made into jewelry or coins (because you don’t really want precious gold particles being lost by rubbing off on your skin and clothes). You often see Olympic athletes bite their gold medals – that’s because traditionally, if you could make teeth marks on the soft metal you knew it was genuine and very close to pure, even though modern Olympic medals are not solid gold. Usually you just need to add a little silver or copper to harden and toughen it.
Twenty-two karat gold is 91.6% Au – if you divide the mass by 24, then 22/24 parts of it by mass are pure gold. Most high quality gold jewelry is made of 22 karat gold. It tends to look a shiny yellow – how bright the yellow is depends on whether the other 2 karats are made from silver or copper. Gold jewelry alloyed with copper is usually has a slightly redder tint. Sometimes zinc is added to the copper to neutralise the rose color and make it more yellow. The best quality 22 karat gold is alloyed with silver (and this is the most expensive version). Because this is the softest form of retail gold jewelry, you need to care for it so that it doesn’t wear thin from over use.
Eighteen karat gold is 75% Au – that is, 18/24 parts of it are pure gold. This is the most popular karat for gold jewelry, as it’s strength copes well with holding gemstones such as diamonds and emeralds in place, without the prongs weakening and the gems falling out.
Most rose gold and pink gold is also 18 karat. Rose gold is composed of 75% gold and 25% copper, and pink gold is 75% gold, 21% copper and 4% silver. It’s the high percentage of copper that produces the rose color. Most antique jewellery is rose-gold, as copper alloys were the most common in the 19th century. Rose gold is sometimes known as “Russian gold” as it was popular in Czarist Russia before the revolution. In modern jewelry it mainly crops up in dual-tone gold jewelry.
Fourteen karat gold is 58.3% Au – that is 14/24 parts of it are pure gold, and the remaining 41.6% are another metal.
Nine karat gold is 37.5% Au – that is 9/24 parts of it are pure gold and the remaining 62.5% is another metal. Because of the low percentage of gold in it, this is the cheapest type of gold jewelry. If the other metal in the alloy is copper, it will look a dull bronze color. If the other metal in the alloy is silver or nickel it will look white, hence “white gold”. European white gold is always 9 karat made with silver because they restrict the use of nickel in jewelry as it causes skin problems.
Because gold is more expensive than copper and silver, the higher the percentage of pure gold a piece of jewelry has, the more expensive it is – 22 carat gold jewelry being the most expensive.
Gold does not tarnish and lasts forever, but the other metals in the alloy will tarnish. 22 carat gold tends to retain it’s shiny yellow look, especially if it is alloyed with silver. However 9 carat gold is vulnerable to tarnishing badly especially if alloyed with copper, and will sometimes turn green or black. The other metals in 9ct jewelry may also react with the wearers skin (especially if nickel is present), sometimes producing dermatitis.
All good quality gold jewelry is hallmarked to tell you exactly what the karat is. 22ct gold is sometimes hallmarked with “916”, which denotes it is 91.6% pure. Sometimes the hallmark will just say “22” to denote the carat. Similar hallmarks are found on 18ct and 9ct gold.