It is a measurement unit which is used for gems.
FL: Stands for Flawless which is eye clean and no inclusions or blemishes are seen through 10X magnification under a microscope.
IF: Stands for Internally Flawless which is eye clean and no inclusions or blemishes are seen internally but might be seen on the surface with a 10X magnification microscope.
VVS: Stands for Very Very Slightly Included which is eye clean and it has very small inclusions that are hard to see with a 10X magnification microscope. It has two degrees VVS1 and VVS2.
VS: Stands for Very Slightly Included which is eye clean and it has small inclusions which might or might not show under 10X magnification microscope. It has two degrees VS1 and VS2.
SI: Stands for Slightly Included which ranges from eye clean to slight visibility. It has small inclusions that range from not seen with the naked eye to visible to the naked eye. It has three degrees SI1, SI2, and SI3 (clean-visible).
I: Stands for Included which has inclusions that are visible to the naked eye. It has three degrees I1, I2, and I3.
Color grade is ranging from D (colorless) all the way to Z (fancy yellow).
There are different shaped from Round to Fancy Cuts like Asscher, Emerald, Oval, Princess, Heart, Cushion, Marquise, Triangle (Trillion), Radiant, and Pear.