Check Hallmark
A piece of gold will usually be stamped with a mark indicating its type. A stamp of “GF” or “HGP” indicates that the piece is gold-plated, not pure gold. In contrast, a pure gold piece of jewelry may show a “24K” or other marking indicating fineness.
The acid test (also known as the touchstone test) is the standard for almost all pawnshops and jewelers in the US, it is second to XRF testing (X-Ray Fluorescence testing) which is far more specialist and expensive. XRF machines can cost $20,000 or more, so you can understand why it's not all too common.
The weight of gold is measured in troy ounces (1 troy ounce = 31.1034768 grams), however its purity is measured in 'carats'. 'Caratage' is the measurement of purity of gold alloyed with other metals. 24 carat is pure gold with no other metals.
Vinegar is very destructive, so when you apply the vinegar to a fake piece of gold, the vinegar will cause the jewelry to tarnish. Gold is a non-reactive metal, so real pieces of gold will not react to vinegar.
Though complicated, fire assay is considered as the most reliable and accurate way to test the purity of gold. However, because of its destructive nature, it is usually used for large quantities or bullion.
Get a dropper and fill it up with white vinegar. Hold your metal object firmly in your hand or set it on a table. Place a few drops of vinegar onto the object. If the drops change the color of the metal, then it is not pure gold.
Scratch test.
If you cut deep enough that you expose underlying metal, you can assume that it is plated. If it appears to be made of a uniform composition throughout, then it is probably solid gold.
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.
Pawn shops use various methods to test gold before pawning. One of the most widely used methods is the acid test, which involves applying nitric acid to the gold item to determine its purity. The magnet test is another simple method used to test if gold is real or not.
Jewelers often use nitric acid to test their gold. Basically, gold won't react to nitric acid, but all other metals will turn greenish.
Goldmeter - real gold detector on the App Store.
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. If that necklace leaps to the magnet, your significant other has some explaining to do.
999” or “1.000” would be pure gold and thus the same as 24 karat. Nonetheless, if an item of jewelry does not have this stamp – it is not necessarily proof positive that the item is made from fake gold. This is particularly true of older items of jewelry. Another method how to tell real gold is with a nitric acid test.
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.
Gold is a relatively soft metal, and consequently gold coins that are made of pure gold or a high percentage of gold, can be relatively soft and malleable. Therefore, biting the coin would leave a mark or an indent if it was 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.
A pure substance is made of constiituent particles that are same in their chemical structure. They have a fixed melting and boiling point and as such the purity can be tested by comparing the melting point of the impure substance with a pure standard.
The Nitric Acid Test
Gold is a noble metal which means its resistant to corrosion, oxidation and acid. To perform this test, rub your gold on a black stone to leave a visible mark. Then apply nitric acid to the mark. The acid will dissolve any base metals that aren't real gold.