Definitions are useful, to a point.
Trying to make a strict distinction between organic and inorganic materials will have to involve making some arbitrary decisions. Coal, for instance, usually contains a few percent clay minerals. One way to weasel out of the semantic muddle you are in is to cite one common definition of an organic compound as one containing both carbon and hydrogen. Diamond does not fall within this definition. It would be a bit more intellectually honest to point out that Nature cares not one whit whether we call a material organic or inorganic.