Black Pearl

is a Reagent resource and also used in the Alchemy skill. It can be harvested from Oyster Beds using the Foraging skill or found as drops from various creatures. You can also purchase.

Source
Black Pearls are found by harvesting oyster beds along shores of ponds and streams. The color of the unharvested oysters is often very close to the color of the nearby ground, making them very hard to see. They can also be obtained by killing NPC Mages and Earth and Sand Elementals in the game.

Currently, harvesting black pearls during the day will only result in one unit per harvest, but harvesting in nighttime hours (Evening through Late Night or about :52 to :07 during Earth hours) will result in multiple units per harvest. The harvest will also result in multiple units of oyster meat.

Trivia

 * Black pearls are a traditional reagent in Ultima.
 * Original description:  They are found in oysters, but ONLY on new moon nights, otherwise the oysters yield some oyster meat and periodic white pearls.