Diamond simulants.

Diamond simulants are a an imitation of a natural Diamond.
Simulants can be natural or made in a lab, and a successful one can be very attractive.
Diamond simulants take a big part in the jewelry industry, for 2 main reasons;
1) Cost of production.
2) Quantity of production.
There are many Diamond simulants in the market this days, and new lines of Synthetic Diamonds ( same chemical composition as a natural Diamond, but grown in a lab)
There are many ways to recognize simulants but don't think that after reading a little online you can detect one, however there are 2 tests that 99% of all simulants fail.
1) Diamond is the strongest natural gem, that hardness makes it possible to obtain extremely sharp facet junction, a simulant on the other hand has a rounded facet junction, and generally shows visible scratches and abrasions.

 look carefully at the facet junction, in a real natural diamond the facet junction is much sharper and better connected.

2) Most Diamond simulants are doubly refractive.

The lines that you see in this syntactic moissanite are a resolute of the light interaction with the simulant.
Diamonds have a very high Refractive index, which is "A measure of the change in the speed and angle of light as it passes from one material to another".
In a natural Diamond the light beam never bends, in most simulants it does.