It can be sold for 4900 (3750 in the Black and White games) at Poké Marts. Big Pearls are like normal Pearls except they are bigger and rarer. These can also be found as a held item by the wild Pokémon: Clamperl, Gulpin, Shellder, and Cloyster. Occasionally, they are found on beaches, but they are much harder to find this way.