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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, or–if the layer of gold is thick enough–goldfilled).
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.

Please note: these tests are for your information only. They are not accurate enough to rely upon when representing a piece for sale. Most jewelers have testing kits to use when determining if something is gold or not and its approximate karat. It is probably easiest to take the item to a jeweler than to set up the testing kits yourself.