10k gold is actually 40 percent pure gold mixed with alloys, so when you want to know what its worth you do this. Say you have 5 grams of 10k gold. Divide 31.1 grams (Thats how many grams are in a ounce)from the gold value on the market (www.kitco.com ) Say today is 900 dollars a ounce. 900/31.1=$28 a gram. Now take 28 x .60(percent of alloys in 10k)=$16.8 . Subtract 16.8 from 28= $11.2 a gram is the true value of 10k which are gold for 5 grams would be worth $56 today.
14k is 58.5 percent pure gold
18k is 78.5 percent pure gold.
Do the same steps for each of these but change the percentage of gold for each when calculating.