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.
In their pure, natural forms, gold, silver, aluminum, copper, brass, and lead are not magnetic. This is because they are all weak metals.
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.
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.
It is possible that an 18k gold piece sticks to a magnet. 18k or 18 karat means that a specific alloy contains 75% of pure gold and 25% other metals. Since pure gold is not magnetic, gold itself will not be responsible for the magnetism, but the metal alloyed may be.
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.
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.
Test Gold Using the Float Test
Take a cup or glass, fill it up with water, now bring the gold that you want to test. Drop it into this filled glass. If the gold floats, it is surely not real but if the gold sinks to the end of the glass then it is pure gold. The real gold will sink due to being a heavy metal.
First off, if your "pure gold" is magnetic, then there is iron inside (or maybe nickel). In fact, this is one way to tell if jewelry is actually gold. If it's not attracted to a magnet then there is no iron - but it still might not be gold.
Gold is not a magnetic metal, so if it pulls towards, or sticks to the magnet, it's fake. However, just because it doesn't react to the magnet doesn't mean it is real, as non-magnetic metals are used in counterfeit pieces as well.
On the other hand, a piece may be gold plated, but it will not attach to a magnet because its core metal is non-magnetic.
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.
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.
(Fun fact: Real gold is not magnetic.) Fake gold, on the other hand, will stick to the magnet.
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.
Basically, gold won't react to nitric acid, but all other metals will turn greenish. Don't douse your jewelry in the acid—just put it in a stainless-steel dish and use a dropper to drop or Q-tip to test a tiny section. If your jewel doesn't react to the nitric acid, you have a real gold piece (14k or higher).
Fool's gold is actually iron sulfide, a non-magnetic, inexpensive and abundant material that is a byproduct of petroleum production.
Hallmarks are imprinted numbers on most authentic jewelry items to showcase their karat percentage. You can expect to see a marking of 24K-10K written somewhere inside of a ring or on the side of a bracelet. These markings may also be written as “750” for 18K gold.