Authigenic quartz crystals formed within skeleton walls frequently contain carbonate inclusions which may characteristically differ from the composition of the skeletons. Whereas the skeletons suffered diagenetic alteration, the inclusions in the quartz crystals were more or less preserved and thus may indicate the original composition of the skeletons, provided the quartz crystals formed not too late in diagenesis. Because they generally grew after the aragonite-calcite transformation, original aragonite skeletons are characterized by low-Mg calcite inclusions in the quartz crystals whereas original high-Mg calcite skeletons differ clearly by the Mg-content of the inclusions. There is no difference between inclusions within quartz of original low-Mg calcite and transformed aragonite skeletons, but the former skeletons are well and the latter are poorly preserved.

By this method, it is shown that Paleozoic Stromatopora and Actinostroma as well as Triassic Ammonoidea presumably were composed of aragonite, whereas the shells of articulate brachiopods (Stringocephalus, Terebratula, Rhynchonella, Gigantoproductus) were composed of low-Mg calcite. The skeletons of Crinoidea (Devonian, Muschelkalk, Jurassic) and of Devonian rugose and tabulate corals, however, were composed of high-Mg calcite.