Below are two attempts at making fairly realistic jewels. The first was testing the concept, and the second I think looks pretty good. Any feedback or tips are appreciated! 
Seriously, any response whatsoever. If all you have to say is that they look terrible, say that. But I really need feedback!
They don’t look terrible, your shading is just off. If you give the diamond more white with a glass effect, you’ll be there. Same with the other colors.
Okay I will give you something to say about this image.
The models itself looks quite realistic however the lighting makes them look non realistic.
Try rendering with cycles and you will give the Image a whole new life 
And add a prism effect too. Diamonds give off all the colors of the rainbow.
I think it’s looking much better!
Now its starting to look like something
keep it up!