There are a couple of variables here. If it is a really nice, unique or antique piece of jewelry, it is worth whatever you can get someone to pay for it. However, with the current bad economy, it isn't really a seller's market right now. Many people don't have funds for luxury items.
If you want to find the scrap value (the value of the actual metal in the piece), that's easy. 18KT is 75% gold - 18/24ths = .75. Let's say the current price for bullion is $1400 per ounce. The final value - after refining - for 1 ounce of 18KT gold will be $1400 X .75 = $1050. As I explained in my hub http://hubpages.com/t/eb878 you will be offered a percentage of this, from 35% ($367.50) up to maybe 80% ($840). Don't mail it in - Consumer Reports warns that the mail-in guys are typically only paying between 11-29%. Find a local buyer - see my hub http://hubpages.com/t/e730b for ideas. As a professional in this industry, my opinion is that anything less than 50% is scandalous. 60-70% is pretty good. 80% would be excellent.
Caution: The above percentages are to sell outright. Some jewelers may offer 100% - but only as a store credit. You then get to take the proceeds and buy something else that may have a 400-800% markup. I recently met a client who paid $2850 for a ring that had $150 worth of gold in it. The original store wouldn't buy it back and he couldn't find any other jeweler who would buy it. Unless there is a piece of jewelry you really want, take the cash.