Help with clusters

I'm trying to make something like amethyst clusters. First, I don't know how to make them placable only on "whole blocks", and second, how do I make them generate on top/connected with other blocks? I have a kyanite block being generated in nether as ore, and I want a kyanite cluster to generate connected to them. Also, if possible, could you tell me what are the hitboxes of amethyst cluster? Thanks!