Assertion: sponge's skeleton | /r/MadeOf | aragonite
Facets:
Source sentences:
text: However, a few also have massive external skeletons made of aragonite, another form of calcium carbonate.   
text: They are sponges with a soft body that covers a hard, often massive skeleton made of calcium carbonate, either aragonite or calcite.
text: Some Demospongiae also have massive external skeletons made of aragonite, another form of calcium carbonate.