Because they are formed under immense heat and pressure, each diamond has tiny birthmarks called inclusions.
Like fingerprints, inclusions make each stone unique. It is possible to see some inclusions with the naked eye, but others are difficult to see even with magnification!
The scale used to categorise the clarity of a diamond can be seen below:
A diamond of exceptional clarity has minimal inclusions to interfere with the passage of light through the stone. The less inclusions usually means a more expensive diamond.