I was intending them to be castes of the same creature... I'm not sure. Is there any way for are a creature to change material as it grows? There's not is there. Having Diorite Crabs laying Gneiss Crab eggs would be "normal" assuming the crab was simply a "baby crab", it appears to gain its actual material as it grows. Or something. It'd be possible by making the child name not include the mineral, then the only way you'd know the mineral before it grew (and became useful) would be if it was butchered for its shell or something.
But yeah, there's be rock crabs, clay crabs, ore crabs and gem crabs. Maybe not exactly the same, maybe the gem crabs would be smaller or something. I'll keep brainstorming ideas.
Note, a friend is also learning DF modding so this is going to provide a platform for some of his ideas too. When done, it will hopefully be like the Caramel mod but really intended to be a subsitute for the standard creatures, or indeed Dwarf Caramel. It's going to be its own thing.
Possible inclusions:
Rock Crabs - Lays minerals, quite bulky, slow.
Magmaworm - Hive vermin?
Longfang Spider - Some sort of cave spider inbetween the normal and giant sized. Imagine generic fantasy spiderling sized, used in war to incapacitate enemies.