Although you won't know for sure until you've sent your jewelry off to the professionals, there are several different steps that you can take to help yourself determine whether to send something to the gold buying company or not.
The first thing to look for is a hallmark, or karat mark. Any gold jewelry sold in the United States and Canada will have an engraving or mark indicating whether it is 10K, 14K, or 18K. Although there is always the chance that unscrupulous jewelry manufacturers have fraudulently marked their jewelry, you usually can rely on the hallmark for accuracy.
Another way to test to see if your jewelry is really gold or made of another metal is to hold a good magnet near it. Gold is not magnetic, so if your jewelry responds to the magnet, it is likely not made of gold. Keep in mind, however, that it may just be the clasp or some other detail in the piece that is made of another material; don't rule a piece of jewelry out that you had thought was gold simply because of the magnet test. Inspect your gold-colored jewelry to see if there are any areas that have silver coloring showing through; if you can see silver, then the jewelry is probably gold-plated rather than made of gold. This will have no value when you try to sell it. You can also do a test with an unglazed ceramic plate. If you rub real gold on a plate, it leaves a yellow stripe; rubbing another metal will leave a black streak.
If you are of a scientific bent and have an accurate jewelry scale and appropriate laboratory equipment, you can check the density of your jewelry. Gold has a very specific density of 19.3 grams. If you weigh your gold, then put it into a vial of water and determine how much water has been displaced, you can calculate the density and determine whether the jewelry's material is in fact gold, or something else.
Finally, if you have nitric acid available to you, you can perform the classic acid test. Nitric acid dissolves silver and other metals, but has no effect on gold. If you pour nitric acid onto your jewelry and it leaves no mark, then what you have is gold.
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