Known as "queen of the oceans", pearl, like coral, is a victim of pollution. The name denotes a calcareous concretion with a silvery lustre formed within the shell of certain bivalve molluscs known as pearl oysters around some foreign body. There are two types: natural pearls, formed inside wild oysters, practically impossible to find nowadays, and cultured pearls in which the production of the pearl is artificially induced. Natural pearls have no nucleus, but in the cultured variety the pearl is formed round a spherical bead inserted into the oyster shell. The oyster tries to render this intrusive body innocuous by sealing it off in a cyst of nacre.

Both natural and cultured pearls are a sound investment as their price is never likely to come down; quite the contrary, it continues to rise inexorably.

The finest natural pearls are fished almost exclusively from the Persian Gulf and the China Sea, while the best cultivated ones come from Japan, Korea and more recently Australia.

The price of a pearl depends much on its provenance, colour and iridescent lustre (known as the "orient") perfect shape and above all size. In fact, the most important characteristic in computing the value of a round pearl is its diameter, which should exceed 8mm.

The finest colours are pinkish-white, bluish-white and white- white. Avoid pearls of a yellowish, greenish or brownish tint. Grey, blue and black pearls are to be found in certain volcanic regions such as Tahiti and may be even more expensive that the white variety.

Pearls may also be of different shapes: round, oval, button and drop (often used for earrings) or of an uneven form, which are known as "baroque" pearls.

Pearl is a rare and living substance and should be treated with great care. If you live in a hot climate, your pearls should be washed in sea water at least twice a year; in cooler climates, once is sufficient. They should not be brought into contact with perfume or other jewelry, which might scratch them.

Remember what I said about investments? Well, if you have a daughter who is coming of age, forget about the bank account, spend the money on a string of pearls. In five years time she will appreciate the difference and be eternally grateful.