Trying to isolate the intruding irritant, the oyster secretes a sticky fluid. The fluid hardens, another layer of it is secreted, and the pearl grows. The genuine pearl oyster is the _meleagrina margaritifera_. _Margaritifera_ means pearl-bearing from which comes the name Margaret meaning pearl. Other molluscs may also form pearls, though not usually the varieties served in the months with an “R”. Freshwater pearls come from mussels, of the kind called _unionidae_. _Unionem_ is the Latin word for pearl—also for onion, which like the pearl is made up of layer upon layer. Mary Queen of Scots had a necklace of fifty-two graduated pearls, all of them fetched out of Scottish rivers. Pearls are prized because of the beautiful lustre that glows upon them, pink or even bluish-grey, an iridescence over the basic white. Rarest are the large black pearls, which make a beautiful center drop on a brooch or a necklace. The pearl is hard and smooth in texture, beautiful to see and pleasant to feel. The usual shapes in which a pearl grows are round, button, pear, and baroque (which in this use merely means irregular). The round pearls are used mainly for necklaces, which must be threaded in silk or plastic or other such material; any metal may darken and dull the beauty of a pearl. Button pearls are used in earclips, studs, brooches and rings. Pear-shaped pearls are attractive as pendants. The use of baroque pearls depends upon their shape and size. Pearls are assorted and matched with great care, according to their size, shape, and color. The matching of a string of pearls may be a quest of twenty years. Sometimes a jeweler will hold the pearls until he has a matched necklace, graduated or of equal size; but it is also a challenge to a woman who enjoys jewels to buy a few pearls she can wear in various ways while watching for enough of their peers to form a string. The lustrous inside of the oyster shell, formed of the same material as the gem, is called mother of pearl. A blister pearl is a flattish excrescence that, instead of being inside the soft oyster, adheres to the shell; it may be detached and used. Seed pearls are very tiny pearls, weighing less than a quarter of a grain. For ages one of the most highly prized and priced of gems, the pearl has become less costly not because of changing taste or of successful simulation, but because man has learned the secret of the stimulation of the oyster to make it create a pearl. The best natural pearls come from the Persian gulf and the waters of Australia; but it is the Japanese who have most fully developed the technique of inserting a foreign body in the oyster, so that it then carries on, under its own living power, the process of making a real—but what is called a cultured—pearl. Man proposes and the oyster disposes.