You've certainly seen labels like "10K", "14K", or "18K" on gold jewelry. The K is an extension of "rust", which is a measure of the purity of gold. For pure gold, where the gold content reaches 99.0% - 99.99%, then its value is 24 carats.
However, pure gold has a soft texture, so it must be mixed with other metals such as nickel, tin and copper, to make it stronger when jewelry is made. That's why, then came some choices of rust value for jewelry, such as:
18K gold with gold content 75%
14K gold with gold content 58.3%
12K gold with 50% gold content
10K gold with gold grade 41,7%
The higher the value of carat gold, would be more expensive price. In addition to its purity level, the price of gold is also adjusted to the weight of gold, which is measured in units of grams. The heavier the gold you want to buy, the price will surely be more expensive.