Clarity
 
Nature made diamonds and none are perfect. 

They all contain inclusions. 

Clarity is graded by examination at 10x magnification. 

The purer the diamond, the rarer it is and the more it costs. 

Clarity ranges from Internally Flawless to Included. 

In clarity grades of SI and above, inclusions are not visible to the naked eye. 

Inclusions will be difficult to see in I1, and easier to see in I2 and I3 grades.

Inclusions are not negative.  They are birthmarks left by nature's formation of the diamond millions of years ago.

if cost is an issue:
then what you usually want is a diamond that doesn't show any inclusions to the naked eye.  That means a good I-1 to SI-2.