Not many people may know that carats do not mean the same for diamonds and gold. For gold, it is a measure of purity, on the other hand, in diamonds it's a weight measure. 

The size of a diamond has the biggest impact on its price. The metric carat, which equals 0.20 gram, is the standard unit of weight for diamonds and most other gems. The best cut will display the diamond's brilliance to the best effect. When looking at a stone you need to look from the top down, this way you can see all of the facets and you need to check to see if you can see any inclusions in the diamond. Teardrop diamond is known as pear and the emerald cut is an octagonal shape which is oblong, the marquise diamond is an oval and it has points at the end. But, the pavilion can be cut with 4, 6, or 8 pavilion main facets. There are so many different types of cuts, there is the round, square, pear and cushion. When a diamond is cut it is cut for brilliance and to let the light reflect off of the facets.

Therefore 24-carat gold is fine (99.9 per cent Au w/w), 18-carat gold is 75 per cent gold, 12-carat gold is 50 per cent gold, and so forth.  Weight of gold is usually denoted in sovereigns, grams and ounce or even kilogramme.