Assertion: oyster's pearl | /r/HasProperty | valued
Facets:
value: highly
type: DEGREE
value: before the days of cultured pearls
type: TEMPORAL
value: for the simple reason
type: CAUSE
value: because of their rarity
type: CAUSE
Source sentences:
text: Tahitian pearls, frequently referred to as black pearls, are highly valued because of their rarity; the culturing process for them dictates a smaller volume output and they can never be mass-produced because, in common with most sea pearls, the oyster can only be nucleated with one pearl at a time, while freshwater mussels are capable of multiple pearl implants.
text: Most molluscs with shells can produce pearls, but only the pearls of bivalves and some gastropods, whose shells are lined with nacre, are valuable.
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.
text: Large, perfectly round pearls are rare and highly valued.