Jewelers have to be aware of how pure the gold in a piece of jewelry is. They can accomplish this using a very simple test called the acid test--not only telling if something is made of gold or plated, but how pure the gold is on some samples. The basic idea behind the acid test is that pure gold is completely unaffected by strong acids, whereas other metals are highly reactive. Nitric acid is used in this kind of testing.
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First, the jeweler takes the piece in question and finds a very small, inconspicuous area to make a scratch. The jeweler then places a small drop of nitric acid on the scratch and watches carefully for any changes. Gold-plated base metal will turn green, and gold-plated silver will turn pink. Anything less pure than 14 karat gold will turn various shades of brown, but 14 karat and higher will have no reaction.
To test for higher purities than 14 karat, the jeweler will use aqua regia, which is a mixture of nitric acid and hydrochloric acid. Aqua regia will dissolve all but the purest gold and will turn lesser karat weights different colors. The jeweler will have a comparison chart to look at that will show him what color each purity level will turn when exposed to aqua regia.