Clarity

A diamond's clarity refers to the presence of impurities on and within the stone. When a rough stone is extracted from carbon deep beneath the earth, tiny traces of natural elements are almost always trapped inside. These elements are called flaws or inclusions because they are formed naturally and are unique to each stone. Inclusions may be crystals of a foreign material or another diamond crystal, or structural imperfections such as tiny cracks that can appear whitish or cloudy. The number, size, color, relative location, orientation, and visibility of inclusions can all affect the relative clarity of a diamond. Diamonds with few inclusions are considered particularly rare and highly valued. After carat weight, clarity has the most influence on cost.

A well-cut diamond will optimize the use of light, creating fire and brilliance. Inclusions can hinder how the light reflects, lessening the brilliance and prismatic effect of color in a diamond.

When gemologists inspect diamonds for overall quality, they must use a 10x magnification loupe (a small magnifying lens used to examine diamonds) to determine the size, type, and position of the imperfections.

Experienced graders use a 10X magnification to determine the various clarity characteristics for the professional clarity grading scale.

FL:
 Flawless with no internal or external flaws. Extremely rare and valuable.
IF:
 Internally Flawless. No Internal flaws.
VVS1:
 Very very slightly included 1. Very difficult to see under 10x magnification.
VVS2:

Very very slightly included 2. Very difficult to see under 10x magnification. Slightly more inclusions than VVS1.
VS1:
 Very slightly included 1. Difficult to see inclusions under 10x magnification. Typically can not see inclusions with the naked eye. 
VS2:

Very slightly included 2. Difficult to see inclusions under 10x magnification. Typically can not see inclusions with the naked eye. Slightly more inclusions than VS1.
SI1:

Slightly included 1. Inclusions are visible under 10x magnification, but may not be very visible with unaided eye.
SI2:

Slightly included 2. Inclusions are visible under 10x magnification and may be visible with unaided eye. More inclusions than SI1.
I1 - I3:
 Included 1-3. Inclusions are visible to the unaided eye. Not carried by Brilliant Earth.