Hi - OK. Gold in its purest form - 24k - is too soft to use for jewelry and therefore other metals are added to the gold, such as silver, zinc, nickel and coper to make it harder. The percentage of the other metals determine the karat. 18k gold is made up of 750 parts of gold and 250 parts of other metals.
Concerning the other marks, we will need see a photo. You can attach a photo of the marks in a follow-up submission if you like.
