According to the wiki page for this tag, this is strictly for use with the core crystal object.
The game itself calls the embedded crystals "Core Crystals" and as of now, the tag for these is Chest Jewel which also uses "Core Crystal" in its definition.
However, the "Chest Jewel" tag seems to exclude blades like Dagas, Floren, Gorg, etc. where they don't have the typical embedded core crystal.
Either the tag name should change: core_crystal_(xenoblade) -> core_crystal_(object)
Or the restriction should be dropped and it should be a tag used for both the object and the body crystals.