FL Diamonds
Flawless: No internal or external flaws.
IF Diamonds
Internally Flawless: No internal flaws.
VVS1, VVS2 Diamonds
Very, Very Slightly included: Very difficult to see inclusions with 10x magnification.
VS1, VS2 Diamonds
Very Slightly Included: Inclusions are visible under 10x magnification to a skilled grader.
SI1, SI2 Diamonds
Slightly Included: Inclusions are visible under 10x magnification and may be visible with the unaided eye.
I1, I2, I3 Diamonds
Included: Inclusions are visible with the unaided eye.