The 4 C's
1. Cut
More than any other attribute, it is the Cut and Polish that makes a diamond brilliant and beautiful. A diamond that is well cut and polished reflects light internally and reveals its brilliance back through the top of the stone.
Poor
light leakages from bottom
Fair
minimal light releases through diamond table
Good
some light releases through diamond table
Very Good
exceptional light releases through diamond table
Excellent
nearly all of the light releases to the viewer's eye
2. Color
The finest diamonds are colorless, rare and expensive. On the GIA grading scale below, D-F are considered colorless, G-J near colorless and any grade K or below shows an increasingly yellowish tinge. Beyond the preference for a whiter stone, however, the color of a diamond does not affect its brightness or sparkle.
3. Clarity
Diamonds with fewer flaws (inclusions) are rare and therefore highly prized. In many cases, these flaws don't detract from the beauty of the diamond because they are invisible to the naked eye.
GIA Clarity Grading Scale
FL-IF
Flawless, internally flawless. No flaws internally or externally. Extremely rare and beautiful diamonds.
VVS1-VVS2
Very, very slightly included. Very hard to see inclusions under 10X magnification. Diamonds of excellent quality.
VS1-VS2
Very slightly included. Inclusions usually not visible to the naked eye. Cheaper than VVS1 or VVS2 grade loose diamonds.
SI1-SI2
Slightly included. Inclusions visible under 10X magnification and might be visible with the naked eye. Good value.
SI3-I1
Included inclusions maybe visible without magnification, but an excellent choice for earrings. An SI3-I1 grade can be an excellent value, particularly since the inclusions are hard to see.
I2-I3
Inclusions can be seen without magnification and obvious under 10X magnification. May affect transparency and brilliance.
4. Carat Weight
The weight of diamonds are measured in carats. One carat is equal to 1/142 of an ounce. Most people think of carat weight as the size of a diamond. However, two diamonds of the same weight can look completely different in size. If a diamond is cut deep and narrow, it will look much smaller than another diamond of the same weight that is cut shallow and wide.
This 1 carat rough can yield a .30 carat diamond with excellent cut and extraordinary brilliance, dispersion and scintillation, or...
The same 1 carat rough can yield a 0.40 carat diamond with fair cut that is dull and dark in comparison