For the fog test, try to hold the diamond or ring between two fingers and breath on it with a puff of air. A light fog will form on the diamond because of the moisture and heat in your breath.
If the fog dissipates right away, the diamond is real. If it takes several seconds for the fog to disperse, it is likely a fake diamond.
Diamonds effectively conduct heat and therefore disperse heat quickly.
Apply some strong heat to the stone.
Diamonds are made of incredibly strong material and will be unresponsive to high heat.
To test this, grab a drinking glass and fill it with cold water. Use a set of plyers or fireproof gloves to hold the stone. Heat the stone with a lighter for approximately 40 seconds, then drop the stone directly into the cold water.
If the stone shatters, it is made of weaker components and is not a real diamond. A true diamond will show no reaction.
This method tests the quality and strength of the stone. Because of the quick expansion and contraction of heat, weak materials like glass or cubic zirconium will crack and break. Think of a glass or Pyrex dish you use for cooking. If you pull the dish out of a hot oven and try to wash it immediately, the shock in temperature change may shatter the dish.
Because diamonds are one of the strongest materials on the planet, it will be resistant to such heat tests. Heat will disperse quickly and the diamond won’t be affected by the change in temperature.