Diamonds form in the earth over millions of years. They sometimes grow faster than other crystals and minerals around them. These minerals and crystals then become included within the diamond. The size, position, number, nature, and relief of these "inclusions" determine the clarity of a diamond. The less inclusions within a diamond the rarer the stone.
The majority of these inclusions can only be seen by using ten times magnification.
The rarest diamonds are "Internally Flawless" which means that under ten times magnification a trained diamond grader cannot see any inclusions.
After Internally Flawless are the VVS1 and VVS2 grades. VVS stands for "Very Very Slight" inclusions. These minute inclusions are difficult for a trained diamond grader to see under ten times magnification.
Following VVS are the grades of VS1 and VS2. VS stands for "Very Slight" inclusions. These minor inclusions again can only be seen under ten times magnification but a diamond grader will find them slightly easier to locate.
"Slightly Included" or SI has two grades also - SI1 and SI2. These have noticeable inclusions under magnification and a diamond grader will find them easy to see, but they are generally not visible to the naked eye.
"Included" or I diamonds have, as a rule of thumb, inclusions that are visible to the naked eye. The I grades range from I1 to I3 with inclusions becoming more obvious and dangerous to the durability of the diamond as you go further up the scale. These inclusions tend to dramatically deter from the brilliance
of the diamond.