Story of Gold

When buying jewellery, we are offered pieces in 9ct, 14ct, 18ct, white gold, etc. What is the difference? A question I am often asked.
To start off with, the word carat is a measure of purity. Unlike in the case of Diamonds, where the same word, carat, is a measure of weight. Gold in its pure form is 24ct and very soft, so to make the metal harder, various other metals are mixed with gold to produce alloys of 9ct and 18ct for instance.
Therefore 9ct means that 9 parts, of the 24 (pure gold), are gold and 15 parts are made up of Silver, Copper and lead-free Brass.
In the case of 18ct, 18 parts are gold and only 6 parts are alloy metals.
Now in the case of white gold, we have a different mix. To change the colour of a very yellow metal to a predominately silver colour, poses its own problems. In the case of 9ct, it is very easy. 9 parts yellow gold and 15 parts of a number of silver coloured metals such as Nickel, Cadmium, Palladium etc.
But, in 18ct we have a slight problem. Here we have 18 parts yellow gold and only 6 parts of a silver coloured metal to change the colour of the alloy. 18ct white gold is normally mixed with Platinum, Cadmium or Palladium, but because there are only 6 parts of any of these metals to change the colour of the alloy, 18ct is always slightly yellowish. To counter act this, most 18ct jewellery pieces are plated with another very, very expensive precious metal, Rhodium. This gives the jewellery piece a lovely white (silver) colour and lustre.
To summarise, the lower the carat, the lower the gold content of the alloy and therefore, the lower the price of the metal.
Please feel free to ask any jewellery related question you may have. I shall gladly share my years of experience with you all.