A huge number of myths are associated with diamonds, including those related to ways of determining a fake. One of these is known as the "breath test". It is believed that real diamonds have a high thermal conductivity, so if you breathe on it, condensation instantly disappears. And although this is partly true, this test cannot be considered completely reliable.
The fact is that other gemstones, such as topaz and sapphire, also have high thermal conductivity and can be confused with diamonds. In addition, there are some synthetic diamonds that have similar properties.
Another well-known myth related to the determination of the authenticity of a diamond says that if you put it in water, the stone will merge with the contents. This is actually a myth. The refractive index of light for water and a diamond is different, in a glass of water even the purest stone will be clearly visible.