Assertion: oyster's pearl | /r/HasProperty | black
Facets:
value: naturally
type: MANNER
value: very rarely
type: DEGREE
Source sentences:
text: Pearls may be a variety of colors, including white, pink, and black.
text: Black pearls are very rarely black: they are usually shades of green, purple, aubergine, blue, grey, silver or peacock (a mix of several shades, like a peacock's feather).[citation needed]
text: Before the days of cultured pearls, black pearls were rare and highly valued for the simple reason that white pearl oysters rarely produced naturally black pearls, and black pearl oysters rarely produced any natural pearls at all.