From Left to right: 18ct Yellow Gold, 9ct White Gold and 9ct Yellow Gold
Something I'm asked a lot is: What's the difference between 9ct and 18ct Gold? And the answer is that all golds (apart from pure gold) are an alloy of gold and other metals. The number of Carats (ct) refers to the amount of pure gold that is present in the alloy and the higher the number, the more gold is present and the more expensive the metal.
The amount of gold present also alters the colour of the gold, this is most obvious in Yellow Gold. In the image above you can see clearly that the 18ct Yellow Gold ring on the left has a much stronger yellow colour than the more muted 9ct Yellow Gold on the right.
9ct Gold: contains 37.5% pure gold
14ct Gold: contains 58.5% pure gold
18ct Gold: contains 75% pure gold
22ct Gold: contains 91.6% pure gold
24ct Gold: contains 99.9% pure gold
Gold is an extremely soft, malleable metal and so the higher the Carat of gold the softer the metal is, the highest carats 22ct and 24ct are not considered suitable for stone set jewellery as the metal is too soft. For this reason gold is alloyed with other metals to give it different properties, strength and durability being the main consideration, these other metals also alter the colour of the alloy giving you different shades of gold, i.e Yellow, White and Rose Gold.
Is usually alloyed with white metals such as palladium or silver to give it it's "white" colour, which is actually a very pale yellow. The yellow colour of White Gold increases depending on the amount of gold that is present: 9ct White Gold is the palest white and 18ct has a stronger yellow colour.
Much of the White Gold you will find in high street jewellers has been Rhodium plated and does not show the natural colour of white gold. Rhodium is a precious white metal, similar in colour to platinum and is often used to plate white gold jewellery. However this plating wears off over time and must be maintained.
Sometimes called Red or Pink Gold is usually an alloy of gold, copper and silver and it is the copper content that gives the metal it's "pink" colour.
So for this alloy the lower carat has the strongest colour as it contains more copper, i.e 18ct Rose Gold will contain 75% Gold and may only have 25% copper giving it a less obvious pink tinge. Where as 9ct Rose Gold only contains 37.5% gold and more copper giving the metal a stronger pink hue.