In all cases of predication of the specific difference we say that the species IS the difference, because IS means : "is subsumed to", And indeed, the extension of the difference is either equal to that of the species (which it should be), or larger than it. In both cases we may, therefore, speak of "subsumed".
In this way, like in the other examples, and especially like in the proposition "A human being is rational ", an Ice crystal IS : the further qualification (involving the chemical compound H2O) of the concrete general crystallization law ( In order to be compatible with the IS, the expression should be grammatically adapted, which we will not undertake here).