For another quest you have to travel to Skellige and meet an older men who wants a black pearl as a gift for his beloved women.

Geralt has to dive for this rare black pearl. You have to collect many shells from the ground of the sea until you get one with a black (instead of white) pearl inside. This coast is one spot where you can collect it.

Otherwise you can buy pearls from merchants (but not for the above quest. I had both - a black and a white pearl in my inventory).