Assertion: sea urchin's shell | /r/MadeOf | calcium carbonate
Facets:
value: partly
type: DEGREE
value: usually
type: TEMPORAL
Source sentences:
text: Most shells that are found on beaches are the shells of marine mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin.
text: The shells are made of calcium carbonate and are formed in layers by secretions from the mantle.