Choosing the clarity is a tricky business. People usually choose clarity based on its grade as they are not confident whether the inclusion will ruin the overall appearance.

In fact, the purpose behind clarity grade is to tell you the location, severity of the inclusion and the most important thing is whether the inclusion is visible to the naked eye. No two diamonds with the same grade of VS1 or SI2 are alike. Inclusions in grades of VS1 and above are not visible even under a 10x magnification while inclusions in VS2 are easily noticeable under the same magnification.

The tip here is, as long as it still looks great under the naked eye, you can safely pick it. You can be assured that VS1 and above is safe to choose as they not visible to the 10X magnification and of course invisible to the naked eye unless you are superman.

 For lower clarity grades VS2 to SI2, it is still possible to find a good diamond which has inclusions unnoticeable by the naked eye. And if you are even more adventurous and desperately needs to make your money worth, it is still possible to pick a SI diamond if the inclusion is not really visible.

How do you find a good SI diamond?

Firstly, never never never never never (no better way to emphasize this) choose a diamond with an inclusion smack in the middle of the diamond table. The inclusion will ruin the shine and brilliance as the light reflections are interrupted.

Well, for a start, choose the diamond that has the inclusion at the corners or near the perimeters would be best. Inclusions in the corners/perimeters are rarely visible.

Secondly, clarity grades are based mainly on the size of the inclusion and do not take into account the color and opacity of the inclusion. Hence, look for inclusions which are white or in a best scenario, completely clear.

 Lastly, choose the diamond with the inclusion that cannot be seen by the naked eye. It is perfectly fine if you can see it under a 10X magnification, that is to check on the two points previously mentioned.

See the example: 
Inclusion is less noticable and nicely distributed
D-SI1-Premium Cut-Round Diamond
H-SI1-Premium Cut-Round Diamond
I-SI1-Ideal Cut-Round Diamond

A SI diamond with dark inclusion in the middle
H-SI1-Premium Cut-Round Diamond

A SI diamond with dark inclusion in the middle and noticable at the corner.
I-SI1-Ideal Cut-Round-Diamond

View more from diamond clarity.