What does ct gold mean?

It stands for the word "carat", and is a measure of the gold's purity. The work is also often spelled "karat", with the matching abbreviation kt.

Pure gold is measured at 24 carats, so divide the gold's purity in carats by 24 to determine how much of it is gold and how much is other metals such as copper or silver. For example, gold measured at 18 carats is 18/24 or 3/4 pure gold.

An alternative measure is fineness, usually expressed as a number with 3 or 4 digits. Often but not always there's a leading decimal point. If it's not present the decimal point is implied. Fineness is used to express the purity of any precious metal, not just gold.

If fineness is given with 3 digits, divide by 1000 to get the actual purity; divide by 10000 if there are 4 digits. For example, pre-1964 U.S. coins were mostly made of 900-fine silver, which means they were 900/1000 (= 9/10, or 90%) silver.