Our attraction to pearls is timeless. Among the first gems known to early humans, pearls have grown in value and appeal from accidental finds by fishermen to a multibillion-dollar industry. Their rarity has made them a traditional sign of wealth, filling vaults from Ancient Egypt to Hollywood and gracing many of history’s famous figures.
What is a pearl? A pearl is mostly aragonite crystals, a form of calcium carbonate (CaCO3) which also makes up other marine organisms from coral to sea urchins. Pearls are produced by a variety of mollusks in warm fresh and sea waters around the world. They appear in a wide range of colors, shapes, and sizes
How do pearls form? The mollusk protects its delicate interior by coating foreign objects (usually a tiny piece of shell or parasite) with calcium carbonate, the same substance that lines the inside of their shells. This “seed” grows larger as the mollusk continues to coat it. Among the mollusks that can produce pearls are mussels, oysters, clams, snails, conch, and abalone.
Pearls vary in size from a pin head to pigeon's egg size. The largest ever found, called the Hope Pearl, is 2 inches (5 cm) long, 3.25-4.5 inches (6.5-11.25 cm) in circumference, and weighs 454 carats (about 90 grams). It was named for Henry Philip Hope, one of the owners, who insisted the name be kept as a condition of the sale of the gem (just as he did with the Hope Diamond). It may be seen in the South Kensington Museum in London, the British Museum of Natural History.
Many people think of wild oysters ingesting a grain of sand as the foreign body which stimulates nacre production.
Living in the sea, oysters are constantly ingesting and expelling sand and irritants.
Only when something becomes lodged - like a piece of shell, bone, coral or parasite does the oyster start nacre production.