Question about diamonds on pendants. How do you tell good quality diamonds from bad?

I have my eye on some pendants for my sterling silver chains. The pendants I'm looking at has diamonds in the description, however, the prices on these seem cheap but yet the items look very sparkly. My question is how do I find good quality diamonds for sterling silver chains/pendants? I want to make sure that the diamonds are of good quality, not cheap quality. Thanks.