Gold karats, also known as ct, k, carat & karat depending on who you ask. Most jewellers will mark the inside of the band with a hallmark noting the fineness of gold used.
Gold karat levels of fineness:
8k - 333 - 33.3% Gold content. Minimum standard for gold in Germany after 1884
9K - 375 - 37.5% Gold content. This is commonly sold in supermarket style & mass production jewellery stores as it is the most affordable form of gold jewellery. Its usually a lighter, less vibrant tone of gold compared with higher carat level gold.
10K - 417 - 41.7% Gold content.
14K - 585 - 58.5% Gold content.
15K - 625 - 62.5% Gold content.
18K - 750 - 75% Gold content. The remainder is made up of metal alloys to add strength & colour. This is generally used for high end jewellery & suitable to set fine jewels.
20k - 834 - 83.4% Gold content.
21K - 875 - 87.5% Gold content.
22K - 916 - 91.6% Gold content. Historically the most widely used fineness for gold bullion coins. This is a lot softer than lower gold karats and may not be suitable for all types of gold jewellery.
23K - 958.3 - 95.83% Gold content.
24K - 999 - 99% Gold content. This is the ultimate in gold karats.