From a BBC recipe: "Place the biscuits into a plastic freezer bag and crush them with a rolling pin until some have turned to crumbs but there are still pieces of biscuit remaining." (my italics)
So there is a difference between crumbs and pieces.
But if you are talking about a messy eater eating a biscuit, you are more likely to get crumbs. That is why your sentence, though grammatical, sounds unnatural.