Pearls are formed inside the shell of certain mollusks as a defense mechanism against a potentially threatening irritant such as a parasite inside the shell, or an attack from outside that injures the mantle tissue. The mollusk creates a pearl sac to seal off the irritation.
The most valuable pearls occur spontaneously in the wild, but are extremely rare. These wild pearls are referred to as natural pearls. The majoriity of pearls sold in jewellery are Cultured or farmed pearls.
Cultured pearls come from pearl oysters and freshwater mussels. Freshwater pearls form in various species of freshwater mussels, which live in lakes, rivers, ponds and other bodies of fresh water. These freshwater pearl mussels occur not only in hotter climates, but also in colder more temperate areas such as Scotland. Most freshwater cultured pearls sold today come from China.
Saltwater pearls grow within pearl oysters which live in oceans. Saltwater pearl oysters are usually cultivated in protected lagoons or volcanic atolls.
The ideal pearl is perfectly round and smooth, but many other shapes, known as baroque, can occur.
Keshi pearls often occur by chance and are not considered natural. Keshi pearls are actually a mistake in the cultured pearl seeding process.
Tahitian pearls, frequently referred to as black pearls, are highly valued because of their rarity. The culturing process for them dictates a smaller volume output and they can never be mass-produced because, in common with most sea pearls, the oyster can only be nucleated with one pearl at a time, while freshwater mussels are capable of multiple pearl implants.
The black pearl oysters found in Tahiti and many other Pacific islands
Black pearls are very rarely black: they are usually shades of green, purple, aubergine, blue, grey, silver or peacock
South Sea pearls can be white, silver, pink, gold, cream, and any combination of these basic colours, including overtones of the various shades of the rainbow displayed in the pearl nacre of the oyster shell itself.
South Sea pearls are the largest and rarest of the cultured pearls – making them the most valuable. Prized for their exquisitely beautiful lustre, South Sea pearls are now farmed in various parts of the world including along the remote coastline of North-Western Australia. White and silver coloured South Sea pearls tend to come from the Broome area of Australia, while golden coloured ones are more prevalent in the Philippines and Indonesia.
Pearls come in eight basic shapes: round, semi-round, button, drop, pear, oval, baroque, circled and double bouldered. Perfectly round pearls are the rarest and most valuable shape. Semi-rounds are also used in necklaces or in pieces where the shape of the pearl can be disguised to look like it is a perfectly round pearl. Button pearls are like a slightly flattened round pearl and can also make a necklace, but are more often used in single pendants or earrings where the back half of the pearl is covered, making it look like a larger, rounder pearl.
Drop and pear shaped pearls are sometimes referred to as teardrop pearls and are most often seen in earrings, pendants, or as a centre pearl in a necklace. Baroque pearls have a different appeal; they are often highly irregular with unique and interesting shapes. They are also commonly seen in necklaces. Circled pearls are characterized by concentric ridges, or rings, around the body of the pearl.