The Fog Test: Breathe on the diamond. Diamond disperse heat. If the stone fog up, then it is a sign of a fake diamond.
UV Light Test: Real diamond will shine blue under UV light.
Looking under a loupe
– Inspect the edges of the stone. Real diamonds have sharp edges, compared to fake diamonds, which have rounded edges.
– Real diamonds are generally mounted on gold or platinum. Look for the poor quality metal mount.
Heat It Up! Heat up the stone for 30 second and drop it in cold water. Observe the internal structure with a loupe. Real diamond is able to withstand the reaction.
Drop It In Water! Real Diamond generally drop to the bottom of the water. Floating stones suggests fake diamonds.
Does the stone scratch? Diamond is the hardest on Mohs hardness scale, which mean it will not scratch. Try rubbing the stone on sand paper.