Pearls, that precious gift from the sea, are a symbol of good taste, appropriate and popular, but never more popular than now, thanks to the unwitting influence of America's First Lady, Barbara Bush.

Traditionally, one of the most alluring facets of pearls has been the mystery associated with them. Centuries ago, for example, westerners believed they had medicinal powers and could cure a host of common ailments.Mythological origins included lighting, tears, dragons, dew, rain, sand and worms. Real-life associations focused on Elizabeth I, Empress Eugenie, and now, Barbara Bush.

Wherever she goes, the signature strands of costume pearls go with her, encircling her neck in appropriately understated patrician style. Women have taken note, advancing on stores in droves, demanding pearls.

Even young women, who a few months ago would have recoiled at the thought of wearing something so grandmotherly, are seen bedecked with pearls. The only mystery surrounding pearls these days is how to buy them.

It pays to shop around and to compare quality factors, which include luster, surface perfection, shape, size, color and matching.

- Luster: A pearl's value comes from its ability to reflect and refract light, giving it a distinctive quality called orient. Orient is described as the deep inner glow and shimmering iridescence that seems to come from the heart of the pearl itself, rather than just a surface shine. The higher the luster, the more valuable the pearl. If the pearl appears dull and chalky, it is inferior.

- Surface perfection: Many pearls are marked with tiny dimples, wrinkles or blotches. These imperfections are the marks of poor quality. The more perfectly smooth and satiny the surface, the more valuable the pearl, but the closer the imperfection is to the drill hole, the less it detracts from the pearl's appearance.

- Shape: Some types of pearls, such as Biwa, rice or baroque pearls, are flat, oblong or irregular in shape. They are beautiful and much less expensive. But when it comes to perfect pearls, the more symmetrically round, the more valuable the pearl.

- Size: Size is a matter of preference and purpose, but as a rule, the larger the pearl, the more valuable it is. Measured in millimeters, pearls commonly are less than 8mm. This is due to the lack of larger oysters needed to produce larger pearls. A 2mm pearl is very small. A 9mm pearl is very large. Generally, pearls of the 61/2 to 71/2mm size offer the greatest versatility in terms of wearability.

In necklaces, the other size to consider is length. The most popular lengths are 14- to 16-inch chokers, 18-inch "princess" lengths, 24-28-inch "matinee" lengths, 30-36-inch "opera" lengths and 45-54-inch ropes - longer if desired. A "collar" or "dog collar" refers to two, three or more strands worn tightly around the neck. Strands 24-32 inches long are the most versatile for long-term wear.

- Color: Depending on the water, the type of oyster and a variety of other factors, pearls range in color from white to black, with shades of cream, silver rosea, cream rosea, pink rosea, blue-green, gray and purple in between. White pearls with a slight rosy tint are thought to be the finest. Color is largely a matter of preference or personal style.

- Matching: This is, perhaps, the trickiest of all factors. Since no two pearls are exactly alike, it takes a trained eye and a good deal of time to match pearls closely according to all the quality factors listed above.

In selecting a pearl necklace, a knot between each pearl is considered a better quality feature. While you may see this in both high and low quality pearls, chances are good that a knot between indicates better quality pearls. If nothing else, a knotted strand won't fly apart and roll in all directions if the string breaks.

Pearls will make the perfect Mother's Day gift this May. If the price for cultured pearls is out of the question, look for finely simulated pearls. They are available so round, smooth and lustrous that the only way to tell them from the real thing is by the price tag. Plan to pay the least for obvious pieces of costume jewelry. Channel style ropes of simulated pearls will continue to be a fashion item for some months to come. Have fun wearing them for now, because they will become quickly dated.

Next week: How to care for your pearls.