Just the other day my friend was in a great dilemma. The jewelry she was wearing which she knew was made of gold, was challenged by some one -that it was not real, it was fake. How to find out? Here are the ways:
The rub test:
At the back of your wrist dab a little liquid foundation. Brush a couple of strokes of powder. Rub the metal in question on it. We were told that real gold will show black marks. But since lot of rubbing is involved the skin gets irritated any way, so though this is an easy test, we could not rely hundred percent on this really.
The sink and float test:
Gold is heavy, so if the metal in question is floating it is definitely fake. But then depending on the shape of the jewelry even brass or other metal can also sink. Therefore this is also not a fool proof one.
The Magnet Test:
Bring a magnet close to the gold jewelry in question. Gold will not be attracted by magnet but its alloys may. Again, since gold jewelry do have some alloy we can not be quite sure.
The Pin Test:
Gold is softer and more malleable than anything else. While other fake metals may break, gold will not. So touching or pricking with a pin may give some clue.
The acid test:
If the metal in question is touched by a drop of nitric acid the following results may give us some clue.
If it is pure gold there will be no change. The gold will pass the acid test. If it turns green then it is definitely fake. If it turns kind of green that means it has some alloy which is natural. If there are milky froth it is mixed with silver, or the metal may be vermeil.
Pure gold is 24 karat gold and to make gold jewelry it needs to be mixed with some alloy or impurity. Now each karat is about 4.167 %gold. In that sense a 22k piece is 91% gold, 14K piece is 58% gold. Anything over 10K is considered gold in the USA. Different countries have different acceptance rule regarding gold. In the USA you can not sell anything less than 10K if you want to call it gold.
Even though testing kits are available in many jewelry stores, (they are around $20-$30 in price range) this nitric acid is quite dangerous and I would rather have a professional person or jewelry store do it. When higher karat gold are in question usually Aqua Regia is used instead of nitric acid.
Jewelry stores probably may also help you to find out whether it is a 22 karat gold piece, 18 karat or lesser karat ones as they have gold pieces with known karats. A test done with a touch stone (a special kind of stone) and acid solution may possibly find out the result.
I have written about these kind of tests more elaborately in my web page along with information regarding rolled gold or gold filled and gold plated metals.
Hope this gives you some idea how to find out real gold from fake ones. Again, the best thing when you are in doubt, take it to a good, trust worthy professional jeweler. Usually they are helpful and may show you the result.