Aqua regia, a mixture of one part nitric acid and three parts hydrochloric acid, is used to test higher karat purity through the process of comparison and elimination.
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.
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.
Gently drop your gold item into the water. Real gold is a heavy metal and will not float, so if your gold item floats you know it is not real gold. Also, if you notice rust or tarnishing on the item after being in water, this is also a sign it is not real gold since gold doesn't rust or tarnish.
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.
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.
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.
Watch the gold for color changes like black or green to see if it is fake. 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.
The Float Test
Place your gold piece into the water. If it's genuine gold, then it will immediately sink to the bottom of the cup. Pure gold is heavy due to its high density – 19.32 g/ ml. If your gold item floats or hovers above the cup's bottom – it's fake or plated gold.
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.
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.
Nitric Acid Test
To perform the nitric acid test, rub your gold item on a black stone to leave a visible mark, then apply the nitric acid to the mark. Nitric acid will dissolve any base metals that aren't gold, a real gold mark will remain.
Mix one part hydrochloric acid to 50 parts nitric acid and 12 parts distilled water.
Look for Stamps or Hallmarks in Your Gold Jewelry
Most real gold is stamped with a hallmark that indicates its purity, aka its karat. The karat is the unit used to measure the purity of gold. The higher the karat, the purer the gold. The most common karats you'll see are 10k, 14k, 18k, 22K, and 24k.
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.
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.
Pure gold on its own cannot stick to a magnet. However, if you have an alloy of gold, then it could stick to a magnet. An example of a gold alloy that may stick to a magnet is gold with over 20% of its atoms replaced by iron. In very cold temperatures this alloy of gold may magnetize all on its own.