Straight line method: the refractive index of natural diamonds is quite high, so it is difficult to see straight lines through the bottom of diamonds. On the contrary, most diamond imitations can see a straight line through the bottom.
Star-petal analysis: the hardness of diamond is very strong, so acute angles will be formed between petals, and smooth or irregular gaps will be seen in imitation names (low hardness causes local wear).
Breathing method: Diamonds have high heat transfer ability. Breathing on its surface will make the fog disappear in less than two seconds.
Refractive splitting method: Because of the high refractive index of diamonds, moving diamonds in dimly lit places (in different directions) will make them shine like "light bulbs".
Magnifying glass observation: most natural diamonds contain inclusions, and some inclusions can't be imitated at all. It is one of the more reliable methods to observe under a high magnification magnifying glass (usually 10 magnification magnifying glass).