Is It Real Gold?
Real gold has a hallmark or karat stamp. NO!
Real gold is heavy. Yes, but how heavy and how can I tell?
Real gold will not tarnish. NO!
A jeweler can just look at a piece of jewelry and tell if it is gold or not. NO!
The myths of how to tell real gold from plated or yellow metal are numerous, and most are inaccurate. What is the best way to tell if a piece of gold is real?
The lack of a karat mark (10k, 14k, 18k etc.)
is not indicative of whether your piece is gold or not. In the United States it is against the FTC's regulations for the jeweler to hallmark a piece of jewelry unless it is accompanied by a maker's mark. So, when I first began making jewelry and did not have a trademark of my own, I did not hallmark my jewelry. Now that I have a maker's mark, I hallmark my jewelry with the appropriate metal hallmark and my maker's mark.
A jeweler cannot just look at a piece and tell you for certain whether it is gold or not; the jeweler will do a test to determine it.
To be scientfically accurate a sample of the metal in question must be assayed in a testing laboratory, but the following two tests have been used for many years and often are sufficiently accurate for a craftsperson or the owner of the metal in question.
To answer "Is It Gold?"
With a small file, make a scratch in an inconspicuous spot. While wearing rubber gloves, use a wooden, glass or plastic stick to apply a drop of nitric acid to the filed spot. Observe the reaction. When done, rinse everything well in running water.
If there is no reaction, it's gold.If there's a bright green reaction, it's base metal.If there's green in the scratch, it's a gold layer over base metal (goldplate).If there's a milky reaction in the scratch, it's a gold layer over silver.
To answer "What Karat Is It?"
Determining karat requires a testing kit containing nitric acid, aqua regia, samples of known karat, and a touchstone of slate or ceramic.
The gold object to be tested is rubbed on the stone ("touched") to leave a streak. A parallel line is made with one of the test pieces of known karat. Both marks are flooded with acid and the reaction are observed. When the sample colors at the same rate as the test streak, a match has been made. Nitric acid is used for low karat golds; aqua regia is needed for higher karats.
Where do I get a testing kit?
Most jewelry supply houses carry these gold-testing kits. However, it is probably easier and less expensive to take your jewelry to a jeweler and have him test the piece.