its not really the top coat that dulls the shine, its the coating of the stones with topcoat.
what makes a stone shine is the cuts on the surface of it, each one of these flat surfaces reflects the light and thats what gives them the twinkle.
once you coat the stone with something else it dulls thes sharp cuts and makes the stones less reflective.
the best thing to do is to glue the stones on with a little nail adhesive and then top coat around the stones so that the base of them is anchored in the gloss but you dont get any on the facets.