This guide describes how to test a piece of gold jewelry with a stone and testing acid.  Many pieces of antique and vintage jewelry do not have karat marks.  Because antique pieces were often very heavily gold plated (much more heavily than modern pieces), very old plated pieces may not show wear and may appear to be gold when they are not.

Testing stones are normally small in size (in the neighborhood of 3" by 1 1/2"), and acids are usually sold in 10K, 14K, and 18K strengths.  Testing kits with a stone and 3 bottles of acid are relatively inexpensive (under $50).  I find the results to be more accurate than the electronic testing equipment I've tried. 

It's a lot easier to understand how to test with acid if you can see someone do it, but if you practice a number of times using known gold items (pieces that are marked with a karat mark) and the following steps, you should be able to get the hang of it pretty quickly.

1.  Use a loupe to examine the edges of the piece of jewelry to see if there are places (wear points) where the gold has worn away.  If you see wear, the piece isn't gold, and you'll just remove more of the gold plating trying to test it.

2.  If you don't see wear, rub an inconspicuous part of the piece (the shank of a ring or an underneath edge of a pin, for instance) across the stone hard enough to leave a clearly visible mark about 1 1/2" long.  Gold is relatively soft, so you don't want to get carried away and rub so hard that you remove a corner of the piece.  You just want to use firm, even pressure.

3.  Put a drop of the 10K acid near one end of the mark.  If the acid eats the mark, the piece is less than 10K.  You can't tell whether it might be 8 or 9K or whether it's gold plated or gold filled, only whether it is not at least 10K gold.  
4.  If you aren't sure whether the mark is gone, blot the stone with a tissue, and you'll be able to tell if it's gone.

5.  If the 10K acid didn't eat the mark, put a drop of 14K acid a little further down the line you made on the stone.  If the 14K acid eats the line, the piece is at least 10K but less than 14K.  US-made jewelry standard karatage is 10, 14, and 18K; but other countries have at various times used 12K, 13K, and 15K.  Sometimes artist-made (rather than factory-made) pieces come in strange karats if the artist melted scrap gold to get the metal to make the piece, but factory made pieces come in standard karatage.

6.  If the 14K acid didn't eat the mark, put a drop of 18K acid a little further down the line you made on the stone.  If the 18K acid eats the line, the piece is at least 14K but less than 18K.

There are ways to test gold using needles and a set of needles to pin down the exact karat, but for purposes of selling jewelry, knowing whether it is 10K, 14K, or 18K is usually sufficient.

Mismarked gold (10K that is marked 14K, for example) is not a serious problem with commercially made pieces manufactured in the US or imported by legitimate sources, primarily because mismarking is against the law and the law is fairly strenuously enforced.  I do not routinely test marked pieces unless I have reason to question the mark (like wear to the gold on a piece that is marked as gold).  Testing is critical for unmarked pieces. 

If an ebay listing title or description indicates that a piece is gold but there's no picture or description of the mark, I would advise writing to the seller and asking how the piece was tested.  Unfortunately, there are sellers who represent pieces as gold that are actually gold filled, electroplated, or washed.  When a listing indicates that a piece is gold but has a very low price, it is especially important to ask questions of the seller before bidding or buying.