Are these correct? Or what would you say?
(grapes etc.)

rare-grained
dense-grained

Question

Are these correct? Or what would you say?
(grapes etc.)

rare-grained 
dense-grained

Answer

Pelin, what do you think "grained" means? Do you mean "seeds"?

Answer

Pelin, 
It would be easier to answer your question if I knew the context.  Usually when used as an adjective "grain" refers to the appearance of wood, specifically the direction of the fibers and growth rings.
In what context are you using that to describe grapes?  Or what quality of the grapes are you trying to describe?