Pure gold, 24 karat, is too soft to be used as jewelry.  Manufactures alloy pure gold with other metals to add strength.  The most common karatage for gold jewelry is 14 karats, however 18 karat and 10 karat are also available.

Knowing that 24k is pure gold, we can determine that 10k gold is 41.6% pure, 14 k is 58.3% pure and 18k is 75% pure.  However, that is really in theory.  Rarely is the karatage exact.  It is quite common to find items marked 14k actually are anywhere from 13-13.75 karats.  Hence, when selling your old gold jewelry bear in mind that what you have may not be exactly what you think you have.  Only a full blown melt and assay can assure you of the true purity.  This is a costly service and usually reserved for the large dealer market.

Once you know the purity, you will need to know the weight in order ot determine the "full" value of your jewelry.  What you get from a gold buyer will always be less due to the costs of conducting business and providing these services.