between the island that is shaped like a walrus, and the small island just to the right of Dragon Nest, I found a large clam in the middle of that channel with black pearls all around it. The buff from the clam makes it easy to get them. Juse look for the bubbles and youll find the clam.

there are also tree stumps in the redwoods. it's incredibly easy to get like that. i'm not sure how it could be easier

there is a cost in the snow area that has lots of black pearls just off shore. They are easy to see at night.

just hit the stumps all around swamp area or redwood. i gathered more by hitting the stumps with bare hands/fists instead of using tools, but maybe it is just because of server settings. Also silica pearls are easy to collect this way

I used a beaver on the stumps and get 20-25 per stump, with x2 harvest on the server.