Anything less than 10 karat gold cannot be legally marketed as gold in the U.S.
24K gold is pure gold, which is generally not practical for wearing, as it is too soft; 18K gold is 75 percent gold; 12K gold is 50 percent gold; 10K gold is 41.7 percent gold. Most gold items are marked with a karat mark, even though it is not required by law.
European quality marks are numbers that indicate the purity of the item. 999 means the gold is 99.9 percent (or 24K) gold; 750 means it is 75 percent (or 18K) gold; 585 means it is 58.5 percent (or 14K) gold.
The different colors of gold (white gold, rose gold, etc.) have to do with the metals with which the pure gold is combined.
If you are looking for gold jewelry on eBay, ask lots of questions. Inquire about the karat mark that is stamped on the piece, the manufacturer's trademark and the measurements--especially the thickness of the ring band.
Gold jewelry never goes out of style, and for good reason, because gold is as wearer-friendly as it is beautiful. Pure gold doesn't react with other elements to create tarnish, the residue that accumulates on some metals before transferring to your skin as a stain. Sometimes people have allergy or staining problems with metals that are combined with gold, but the gold itself is rarely a problem.
Gold can be worked into nearly any shape, including tiny strands that do not break easily. One ounce of gold can even be hammered into an ultra thin sheet that's ten feet square. Gold can be manipulated nearly any way the artisan desires.