In order to distinguish gold from other base metals, the gold-containing item is rubbed on a black stone/surface which will leave a visible mark. Nitric Acid is then applied to the mark. If it easily dissolves, the item is not Gold. If the mark remains, Aqua Regia (Nitric Acid and Hydrochloric Acid) is then applied.
The most trustworthy method to test if something is gold plated or solid gold is to have an acid test done it. For the acid test, take a small piece of jewelry, or using a small needle scratch a tiny part in your gold, take a dropper given in the acid kit, fill it with acid.
In a nitric acid test, a jeweler places a few drops of the acid on the piece and watches the reaction. With copper, the reaction usually causes the liquid to turn green, give off fumes, and sometimes bubble. This response is known as a hard cupric reaction and indicates that your piece is not real gold.
A solution of Stannous Chloride called “Purple of Cassius” has been used for centuries by gold prospectors to test for the presence of gold in a solution.
The Acid Test
Drop a small amount of liquid nitric acid on that scratch and wait for a chemical reaction. Fake gold will immediately turn green where the acid is. Gold-over-sterling silver will become milky in appearance. Gold will not react to the nitric acid.
Fill a cup or bowl with water and carefully drop your gold piece into it. If the gold is real, it will sink to the bottom of the cup. If it's fake, it will float to the top or hover in the middle of the cup.
Make a small scratch on the gold and add a drop of bleach. If it changes color, your jewelry is not 100% pure 24K gold. Bleach can permanently damage gold jewelry. It can't tell you if your gold is fake, since most gold jewelry has other metals mixed in to give it strength.
What to do: Hold the magnet up to the gold. If it's real gold it will not stick to the magnet. (Fun fact: Real gold is not magnetic.) Fake gold, on the other hand, will stick to the magnet.
The acid test is the most common method used while pawn shops test gold. This test involves applying a small amount of nitric acid to the gold item to determine its purity. If the gold is genuine, it will not react with the acid, and there will be no change in the color of the metal.
Acid test.
An acid test can reveal the karat of solid gold jewelry, but it can also show whether jewelry is solid gold or gold-plated. With an acid test, a small sample of the jewelry is removed and exposed to acid to induce a color change. The resulting colors indicate which type of metal the jewelry is made of.
The apple cider vinegar gold test is a great choice if you want to test your jewelry at home, as it is readily available and easy to do. After all, real gold will not react when exposed to the acids in vinegar, toothpaste and more.
Check if your gold is real by performing the magnet test. Real gold will not attract a magnet. To test if 18k gold is real, hold it next to a magnet. If the magnet sticks to your jewelry, then it does not have a high percentage of gold but is made up of other, more magnetic metals.
The acid in the vinegar can damage these delicate stones, leaving your jewelry looking less than its best. If you have gold jewelry with stones and want to clean it well, use mild soap and lukewarm water instead.
If your gold piece turns black or green when the vinegar is on it, or if it starts to smoke or fizzle at all when the vinegar touches it, it is most likely not real gold. If your gold piece does not change colors and does not fizzle or react to the vinegar in any way, it is probably real.
Apply concentrated acid to the metal. Brass will react with acids and gold will not. If you see bubbling or discoloration where the acid is applied, your piece is brass. If there is no change after applying the acid, you have gold.
To perform the test, first, scratch a small area of the gold with the file or sandpaper. This will remove the top layer of gold, exposing the inner layer. Next, apply a small drop of nitric acid to the scratched area. Observe the reaction, and compare the results to the chart provided in the kit.
Black Gold Testing Stone – you must thoroughly rub it with a 320 grit sandpaper, wash and rinse it before making each test.
The toothpaste will not affect real gold, but impure gold will change color. The black streak that appears is due to the presence of other metals, like copper or silver. Keep in mind that this test will not work on gold-plated items. The toothpaste will remove the thin layer of gold, revealing the metal beneath.