Assertion: oyster's pearl | /r/HasProperty | black pearls
Facets:
value: frequently
type: TEMPORAL
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.