Never assume it is Natural Diamond even if it is. Always assume it is Synthetic unless you can prove otherwise.
What is a “Synthetic” Diamond?
Technically, a synthetic diamond is a man-made diamond. One that is created by artificial, rather than geological, means. The term “synthetic” encompasses many other terms related to diamonds including “man-made,” “lab-created,” “laboratory-grown,” “cultured diamond,” and “cultivated diamond.” These diamonds, like natural diamonds, are made from carbon and have the same refractive index, density, hardness, dispersion, and crystalline structure as those diamonds mined from the Earth. One of the most common methods of creating synthetic diamonds is Chemical Vapor Deposition (CVD). Synthetic diamonds are often confused with another non-natural diamond called a “diamond simulant”.
What is a Diamond “Simulant”?
A diamond simulant only looks like a diamond but they are very different. The density, refractive index, and dispersion are all different compared diamonds. Diamond simulants are far less expensive than both natural and synthetic diamonds. A few examples of diamond simulants are Cubic Zirconia (CZ), Synthetic Moissonite, high-leaded glass (rhinestones), white sapphire, Yttrium aluminum garnet (YAG) and other white transparent stone including glass and plastics. All the simulants earned a softer rating than the diamond’s “10” rating on the Mohs Scale.
You can visit http://www.gia.edu/identifying-lab-grown-diamonds for more info.