What is Moissanite?

After many decades of attempts, it was successfully synthesized in a laboratory and is now the modern-day lower cost alternative to diamonds, even exceeding diamonds in light reflectance!
Moissanite color ratings:
Colorless: D, E, F.
Near-colorless: G, H, I. J.
It continues from K to X, but those grades are rarely used in jewelry.
There are no A, B, C nor Y, Z grades.

It also has a clarity (flaw) grade...

Moissanite is rated 9.25 on the Mohs hardness scale. Diamond being 10.
It only became available to the jewelry trade in 1998, so is still a relatively unknown gem.
Due to the high complexity in growing Moissanite, less than a handful of companies produce it.