Most people are confused by the difference between 9 carat, 18 carat and 22 carat gold. They know that the higher carats are more expensive, but they are often not clear why.

This page explains how gold carats are used to indicate how much gold there is in a piece of jewelry.

Gold carats are always denoted in twenty-fourths. Thus pure gold has 24 carats - that is 24/24 parts by mass is gold. You tend to find pure 24 carat gold only in gold bullion slabs. You never get 24 carat jewelry because it is so soft the particles would rub off on your skin the more you wore it, and it would thin after prolonged usage.

To make jewelry suitable for wear and tear, gold is blended with copper, silver and sometimes nickel to harden it. The carats tell you how much other metal has been added.

Nine carat gold is 9/24 parts gold by mass, or 37.5% gold. This means that 62.5% of the jewelry is another metal.

Fourteen carat gold is 14/24 parts gold by mass, or 58.3% gold. This means that 41.6% of the jewlery is another metal.

Eighteen carat gold is 18/24 parts gold by mass or 75% gold. This means that only 25% of the jewelry is another metal.

Twenty-two carat gold is 22/24 parts gold by mass or 91.6% gold. This means that the jewelry has only 8.4% other metal. Twenty-two carat gold jewelry is the most expensive as it is nearly pure.

The cost of the jewelry depends not only on the how much gold there is in it, but the cost of the other metals added. Copper is cheap, therefore gold jewelry made with copper is cheaper than gold jewelry made with silver.

Gold is a bright shiny yellow metal. The more you add other metals the duller it gets. You can also tell by looking what other metals have been added.

22 carat gold jewelry will be a bright yellow. Mixed with copper, 18 carat gold will be less bright and 9 carat gold will be dull. If the alloy is with silver or nickel, the colour pales, so 18 carat gold mixed with nickel will be a pale yellow, and 9 carat gold mixed with nickel will look white (and is marketed as "white gold"). Most white gold in Europe is made with silver rather than nickel, as nickel causes dermatitis in some people. 9 carat gold made with silver will be more expensive than 9 carat gold made with copper.

Don't just rely on inspecting the jewelry to tell the carats in it - all good jewelry will be hallmarked by the jeweller to indicate the carat.

Gold is a famously inert metal - that is it doesn't react with any other chemicals. However, some of the other metals used in the alloy do react, especially copper, which corrodes. Thus people with 9 carat gold jewelry made with copper may find that the jewelry tarnishes and goes green or black, leaving marks on your clothes and skin. Therefore to retain the colour, go for the highest carat gold you can affford.