South Sea pearls are the largest cultured-pearl variety, sometimes reaching over 20 millimeters in diameter. The Pinctada maxima oyster can take up to five years to produce a single South Sea pearl, whereas more common freshwater oysters can take as little as three months and create dozens of pearls. This long cultivation process makes South Sea pearls rarer — and more expensive. A single South Sea pearl can cost $1,500, and a necklace can reach over $200,000. So, how are these pearls grown? And what makes them so expensive?