Carat in regards to diamonds and all other gem stones is a measure of weight, unlike Karat for gold which is a measure of purity.
As I’m sure you know there are a lot of different cuts for stones. The cut of a diamond is not just about the shape. Every cut has an “ideal cut”. Most diamonds are well cut and vary only slightly from the “ideal cut”, but few are a perfect cut. Flawless stones or” ideal cut” are very rare.
The element symbol for a flawless white diamond is simply “C” for pure carbon crystals. Diamonds are graded for color in two different ways, the normal color range and the fancy color spectrum. The normal color range of diamonds is from colorless (D) to hints of a warm yellow to brown and on down the alphabet to black. D is perfect and colorless, then of course an E color would have a slight difference and on down the alphabet. Fancy colors are fairly rare. An intense Canary yellow diamond is a completely different color of yellow than a Champaign yellow. Again, the more rare, the more valuable.
It is EXTREMELY rare to find a flawless diamond. To get a “flawless” grade, your diamond must have absolutely not even a microscopic spec of imperfection inside the stone and also have a perfect “ideal cut”. The next highest grade is internally flawless, which means it is one of the very few stones that have no internal flaws but may have a cut that is not ideal.