The number refers to the fineness of gold in parts per 1000. All these marks are acceptable as it depends on where it was made. There are many carats used in jewellery. Below are the most common?
8K, 8ct or 333 (parts per 1000) or is 33.3% gold (usually European. Not recognized in USA. The lowest possible standard. Will tarnish sometimes)
9K, 9ct or 375 (parts per 1000) or is 37.5% gold (mainly British Commonwealth countries. Not recognized in USA. Hard wearing.)
10K, 10ct or 416 (parts per 1000) or is 41.6% gold (Minimum USA standard. Hard wearing.)
12K, 12ct or 500 (parts per 1000) or is 50% gold (usually old watch cases. Rarely seen)
14K, 14ct or 585 (parts per 1000) or 58.5% gold (Asian for overseas market. Common in USA. Russian 584.)
15K, 15ct or 625 (parts per 1000) or 62.5 % gold (British Commonwealth countries. Discontinued c1935. A good indicator that the piece is old.)
18K, 18ct or 750 (parts per 1000) or 75% gold. (Most quality pieces are 18ct. Minimum gold standard for sale in Italy.)
22K, 22ct or 916 (parts per 1000) or 91.6% gold (mainly Asia, Middle East. Very soft. Very yellow)
24K, 24ct or 1000 (parts per 1000) or PURE gold (too soft for most jewellery manufacture)
