As I have already said, I don't think a blade of grass or a piece of sand, or a small rock would be good foci, for the same reason that dead skin cells almost certainly aren't. They are too small. I might be wrong, yes, but I don't think I was wrong to ask the question here. We learned valuable things, and lost basically nothing.
@DragonParadox could you clarify a bit: when Demonreach asked us not to use the Crown again, did he mean "on the topics related to the island" or did he mean "while Molly is physically on the island?"?