On jewellery, ct is the written abbreviation of carat when we are talking about how pure the gold is, not how heavy a stone is. Pure gold is 24 ct: 24 parts gold out of 24. An 18 ct gold necklace is 18 parts gold mixed with other metal, so it is stronger and a little less yellow. British shops stamp 9 ct, 18 ct, 22 ct and so on into the gold. The example you want in your head is "a 24 ct gold necklace". Though actually, Taiwan shops almost never write ct for this. They write K, from the American spelling karat: 18K, 24K, 24開金. Same ladder of purity, different letter on the tag. And in another corner of the jeweller's window, ct on a diamond is weight (克拉), which is a different use of carat. This entry is the gold-purity one.