Help with spawning a certain block on Surface/Grass only

So , i started working on a mod today, it's supposed to make you very OP(Overpowered) in no-time...

(i know it's easy then, but it's made for me and other ppl who struggle on survival)

My main question here is, i made a grass Diamond(ore) block  that gives you a block of diamond. but i want it to only spawn on the surface/grass blocks.

I have no idea how i would be able to do this, i researched it but couldn't find anything about it.

Any help would be appreciated.

(PS: sorry for spelling mistakes)