Most of the natural diamonds are created at high-temperature, high-pressure conditions within 140 to 190 kilometers of the mantle of the Earth. Minerals with carbon provide the diamond’s source of carbons as it grows between a billion to 3.3 billion years. Deep volcanic eruptions bring diamonds closer to the Earth’s surface, eventually cooling and turning into igneous rocks called lamproites and kimberlites. A diamond can also be synthetically produced via a high-temperature, high-pressure process that simulates conditions in the mantle. A completely different technique for creating diamonds is called chemical vapor deposition. A number of non-diamond materials like silicon carbide and cubic zirconia resemble diamonds in properties and appearance so they are called diamond stimulants. Consequently, there were also techniques developed for differentiating diamond stimulants, synthetic diamonds, and natural diamonds.
According to the LJWest website, you can identify “natural” diamonds because their thermal conductivity levels are high. They also have a high refractive index, although there are other materials that can compare. A diamond can cut glass but this is not enough to identify diamonds because there are other materials that can also cut glass, like quartz. Diamonds can also scratch other diamonds but this simply sometimes results into damaging both stones. Hardness tests are not usually done because they can damage the gems during a test. Extreme hardness and the high value of a diamond means that the gem stone was polished slowly using traditional techniques with greater attention to detail than what would normally be given to other gem stones, resulting into the highly polished, extremely flat, and sharp facets of a diamond. When taken together, all factors play a role in affecting the overall appearance of a diamond. Sometimes, the only real way to determine a diamond is to look at it very closely, relying on the skillful use of a loupe to identify the gemstone by eye. This is why you’ll still see a lot of jewelers always have a loupe on their person.