No, gold will not turn black when you burn it with a lighter. If your gold jewelry turns black when you hold a lighter to it, it's most likely fake.
Heat your gold jewelry with the lighter.
Watch it closely for any changes in color. Fake gold will get darker, while pure gold will do the opposite: it will get brighter the hotter it gets. The brighter your jewelry gets, you can rest assured you have real gold.
Place your jewelry on a table or hold it in your hand, pour some white vinegar on the metal directly (a dropper can also be used) if the metal of the jewelry changes its color, it is not pure gold and if it keeps shining then you have real gold in your hand.
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.
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.
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.
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.
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.
Get a ceramic plate and scratch the gold piece you're testing against it. If the gold is real, you should see a yellowish streak on the plate. If it's fake, you'll see a dark streak.
Reflectivity: Gold is a highly reflective metal, reflecting both light and radiation. Gold reflects infrared radiation with wavelengths above 0.7 µm as well as any other metal, keeping electronics cool. It also reflects more UV radiation below 0.35 µm than other 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.
An excellent way to distinguish brass from gold is to use the acid test. Put one to two drops of nitric acid on the product you want to test. If there is no reaction, the product is gold. If you see a green, smoky, and fizzy reaction, there is likely copper in the gold you are testing.
Goldmeter - real gold detector on the App Store.
Test With Nitric Acid
Make a mark deep enough to scratch through the top layer of gold. Carefully apply a drop of nitric acid to the mark, and determine if the mark turns green or milky. There will be no reaction if the jewelry is either gold or mostly gold.
In their pure forms, gold and silver happen to be very soft metals—soft enough that you should be able to mark them with your teeth. According to the Mohs hardness scale—which relates pairs of materials according to which one will scratch the other first—gold scores a 2.5 and silver, which is harder, a 2.7.
Toothpaste can damage your diamonds, gemstones, gold and silver. Toothpaste is abrasive and has a hardness of around 3/4 on the Mohs Scale of Hardness. Metals such as gold and silver are softer, so toothpaste can actually scratch damage your gold and silver jewellery.
Because gold is a soft metal, it is easily scratched even with a light abrasive like baking soda.
In conclusion, testing gold with alcohol is a simple and easy method that can be used to determine if an item is made of solid gold or if it is gold-plated. However, it is important to remember that this method is not as accurate as acid testing and should only be used as a preliminary test.