South Sea pearls are grown in the Pinctada maxima oyster, the largest pearl-producing oyster in the world. They are known for being the largest of all cultured pearls, typically ranging from 8mm to 16mm in diameter, and can even reach up to 20mm. They have a soft, silky luster, a result of their thick nacre (the pearly substance that makes up the pearl). Don't worry about the little nobbly bits at the top. They are lost when set.
