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.

A, B, C so why D

As to the famous question of why the Diamond normal color scale starts from the letter D instead of starting with the letter A, there is a very interesting reason with a brilliant outcome.
Imagine you have a colorless Diamond, and you call it "A" color, your friend who happens to have a colorless Diamond to, decided to call his "AA", that immediately makes your Diamond a  non"AA",  the next day when a third friends decided to call his colorless Diamond "AAA" all the other colorless Diamonds become not so colorless because they are no "AAA".
In other words the GIA had to start a new system that had no connection to any existing system, in order to avoid confusion, and this is how the letter D become the first one to start the scale.
Some people say that the reason was to leave an open space in case a new color is discovered, the GIA color grading people realized that if they want to enter history they have to leave open door to possible future discovery of new colors.
So just in case anyone tries to sell you and A B or C color diamonds, tell him you found an "AAA" Diamond for a better price and find your way out as soon as possible.