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.

On jewellery在珠寶上, ctct is the written abbreviation是carat的書寫縮寫 of carat when we are talking about how pure the gold is指的是黃金的純度, not how heavy a stone is而不是一顆寶石有多重。Pure gold純金 is 24 ct是24 ct: 24 parts gold out of 2424份裡有24份是金子。

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".

An 18 ct gold necklace一條18 ct的金項鍊 is 18 parts gold mixed with other metal是18份金混著其他金屬, so it is stronger and a little less yellow所以比較堅固，顏色也沒那麼黃。British shops英國店家 stamp會印上 9 ct, 18 ct, 22 ct and so on9 ct、18 ct、22 ct等等 into the gold在金子上。The example you want in your head你腦中要記住的例子 is "a 24 ct gold necklace"是「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.

Though actually不過其實, Taiwan shops台灣的店家 almost never write ct for this幾乎不會為了這個寫ct。They他們 write K反而寫K, from the American spelling karat源自美式拼法karat: 18K, 24K, 24開金18K、24K、24開金。Same ladder of purity同樣一套純度的等級, different letter on the tag只是標籤上寫的字母不同。

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.

And而且 in another corner of the jeweller's window在珠寶櫥窗的另一個角落, ctct on a diamond用在鑽石上時 is weight (克拉)指的是重量（克拉）, which is a different use of carat這是carat的另一種用法。This entry這個詞條 is the gold-purity one講的是黃金純度這個意思。