Keep in mind, that often the difference between a $2,000 diamond and a $4,000 diamond for example could be small imperfections that can only be seen under 10X magnification.
This explains the ratings for one of the C's, Clarity.
2 guys can each buy diamonds for drastically different prices with the only difference being that one is a VVS1 and the other is a SI2 but both look identical to the naked eye.
The thing to be aware of is that depending on where the inclusion is it may or may not affect the way light is reflected. Be careful to avoid fractures unless you know that it won't spread causing the diamond to crack.
Diamonds are rediculously overpriced at an inflated value as it is, no sense paying $1-2K extra so that if someone pulls out a loop they won't see a small black dot in it!