It used to be basic black and pearls, remember? Now it's basic pearls with everything. Pearls have become a favorite accessory with sweater-dresses, suits and, of course, black dresses.

Ralph Lauren showed graduated pearls with his old-fashioned lace-collar dresses and handknit sweaters, Bill Blass wrapped the throats of his models in rows of pearls.And just about every designer is showing black dresses or suits for fall -- with at least one strand of pearls.

While some designers mix jewels with pearls, and some use tiny-size pearls, the best investment may be the medium-large (6 1/2 to 7 mm.), in the 24- to 30-inch length that sits proudly on a jewel-neck blouse, sweater or dress, or outlines the decolletage of a V-neck style.

Pearls come in three varieties:

Real -- or "oriental," are formed naturally in oysters, and only look different from the cultured variety in an X-ray.

Cultured -- pearls raised in Japan in oyster beds, taking from four to seven years to "grow," but just as "natural" looking as the so-called "real" pearls. Both real and cultured fresh-water pearls which come from mussels, are usually oval and "baroque," uneven textured. The fresh-water variety can be found in the United States in the Mississippi and Missouri Rivers. They've even tried the Chesapeake Bay but without much success.

Strictly fake -- mostly right out of the chemistry labs and paste pots. The best of them are coated with an iridescent mother-of-pearl found in fish scales. The more coats, the more iridescence, the more expensive the "pearls."

Since the price range can be from $200 to $50,000 and more for fine-quality cultured pearls -- depending on size, length, quality and clasp -- it's essential to find the best quality in the price range you can afford.

Charles Sherman, co-founder of Charles Ernest, which carries Mikimoto pearls in Whshington (Mikimoto has the largest oyster beds in Japan), offers these guidelines to quality:

Size -- Larger pearls are more expensive, simply because it takes longer to grow them. (Mikimoto pearls are 7 years old.) The longer the pearl is in a living oyster the more nacre (or mother-of-pearl) the oyster has thrown off. For a uniform strand, there shouldn'd be more than 1/2 mm. difference between pearls.

Don't expect matched pearls, however, to match exactly, says Vogue magazine. "Oysters are independent contractors, not factories."

Surface -- Expect natural imperfections such as little bumps and tiny recessed pinpoints.

Weight -- Heavier is better, according to Sherman, because it indicates the pearl is older and has more nacre. Customs duty paid on pearl imports from Japan is based on the weight of the pearl.

Luster -- Pearls should have a clarity of surface so that you can almost see your reflection in them.

Color -- Cream rose is considered by many to be the most flattering, although some people prefer a pinker tone. Pearls take on a "hue of wearer," says Sherman, and improve with age. Years ago pearl cultivators paid to have pearls worn for five or six months, then returned, cleaned and sold.

Sherman, asked once to clean Jacqueline Kennedy Onassis' seven-strand necklace of cultured pearls, was amazed at the color.He asked her secretary if she put on hair spray after being totally dressed, including her pearls. She did. The hair spray had dulled -- almost changed -- the color of the pearls.

Pearls, for that matter all jewelry, says Sherman, should be the last thing a woman puts on before going out, after the hair spray and after the spray perfume, which is equally damaging. Pearls need occasional cleaning with a mild Ivory soap solution, then towel drying, and if worn often should be cleaned and restrung professionally at least every couple of years. Once a year would be best, he says.

The current pearl boom is considered by some a reflection of a search for familiar and safe luxuries in the face of inflation. The demand, however, has helped to boost prices in a stable or diminishing market.

According to Sherman, the value of top-quality large pearls has doubled in the last nine months, more even than gold. But not more than the finest diamonds, which have escalated four times in that period.