Diamond field identification: what actually works, and what does not

Diamond is a 10 on the Mohs scale and will scratch essentially anything else, but hardness alone cannot confirm a diamond since being scratched by a diamond only shows the tester was harder, not that the tester is a diamond specifically, and there is no material a diamond simulant can reliably scratch that a diamond cannot also scratch. Popular home tests like breathing fog onto the stone to see how fast it clears are unreliable folklore rather than genuine science, several simulants behave similarly enough to fool this test. The properties that actually separate diamond from simulants (thermal conductivity, specific gravity, and optical properties) require dedicated testing equipment or a trained gemologist, there is no reliable field trick that replaces that.