The vast majority of diamonds contain some impurities, known as inclusions, which can detract from their beauty and impede light return. The size, number, type and location of these inclusions all factor into the final diamond clarity grade. The universal system for grading diamonds is the F (flawless) – I3 (inclusions III) system as noted below:
F AND IF
F means internally and externally flawless. 1F means internally flawless, a scratch on the surface of the diamond may need to be removed. F and 1F diamonds are very rare and expensive.
VVS1 AND VVS2
VVS means very, very slightly included. This means it is very difficult to see the impurities, even using a 10-power loupe. The numbers represent level within this grade- VVS1s are cleaner than VVS2s.
VS1 AND VS2
These diamonds are very slightly included with impurities that are not visible to unaided eye. VVS and VS diamonds are known as "Eye-Clean" which means either a microscope or a 10-power loupe is necessary to see them.
SI1 AND SI2
These diamonds are slightly included which means the inclusions are visible under 10-power magnification and might be visible to the unaided eye. Sometimes, a diamond is graded SI1 even though it has no more inclusions than an VS2, but because those inclusions occur in the centre of the diamond, they are more noticeable and more likely to block light. Some inclusions can be hidden by a mounting, thus having little effect on the beauty of a diamond.
I1, I2 AND I3
These diamonds contain visible inclusions.