A diamond's clarity is determined by the degree to which it is free from naturally occurring inclusions often called 'nature's fingerprints'.
The number, nature, size and position of inclusions are assessed using a 10x lens.
Inclusions absorb light and reduce the brilliance of the diamond. Unfortunately, many inclusions are too small to be seen with a 10x lens, so they are not counted for certification purposes. However, these micro-inclusions still adversely affect the brilliance of the stone. As a consequence of this, two diamonds with identical certificate specification can be very different when compared visually – one might have amazing life and fire, whilst the other, with it’s micro-inclusions may be dull and lifeless.
Clearly, it is unwise to buy a diamond on the basis of a certificate specification alone – it is essential to see the actual stone.