You've worn a gold ring for quite a while, but this time when you take it off there's a black smudge on your finger.

What's wrong?According to Jewelers of America, Inc., a national trade association in New York City, more than 99 percent of the time smudging is caused by something other than the quality of the gold.

Cosmetics are the most common cause of the problem, according to the association.

Known as "black dermographism," which literally means "black writing on the skin," this phenomenon occurs when the cosmetics contain chemical compounds harder than the jewelry metals they contact.

The most common of these compounds are zinc oxide, titanium dioxide, ferric oxide and calamine.

Zinc oxide is a major white pigment used in face powder, creams and lipstick. It has recently been replaced by titanium dioxide, which can be equally hard on jewelry.

Rouge and face powders often contain ferric oxide pigments. And calamine, a tinted oxide, is found in some medicated cosmetics and lotions.

Particles from these cosmetics, when left on the skin, act like fine sandpaper.

They abrade the softer jewelry metals - gold, silver, platinum and nickel - and leave deposits on the skin.

These deposits look black because the particles are too small to reflect light. You can remove them with soap and water.

Sometimes other skin discolorations are found particularly under rings.

Some people have sulphur in their perspiration that produces a greenish-black stain when it contacts any metal jewelry.

Such people should remove rings often and use an absorbent powder on their fingers that's free of zinc oxide.

Also, soap, detergent, wax, polish or cosmetic cream can build up under a ring, causing a type of dermatitis.

A burning sensation and redness of the skin results. To prevent this, simply remove rings before using soap or detergent and clean the rings frequently.

Certain rings, especially those containing copper and silver alloys, will leave a black smudge on the skin if enough salt is present.

Simply exposing the hands to salted nuts, crackers or popcorn can cause such corrosion and thus discolor skin. To avoid this, rings should be taken off when hands are exposed to salt.

Actual allergies to silver, gold and platinum are really very rare. If you switch to a different alloy you can solve the problem since most reactions are to the alloy and not the base metal.

For example, jewelry containing a nickel alloy such as white gold can cause a black smudge. If a nickel allergy is causing a reaction, switch from white to yellow gold. This will elminate all nickel.

Discoloration is not the jeweler's fault. It's not the customer's either. Sometimes the jewelry doesn't agree with its owner.

Regardless of whether the reason is cosmetics or even allergies, salt or sulphur, there is a cure for each type.

When a consumer purchases jewelry from someone they know and trust and who is established in the community, then it is easy to remedy the problem.

For more information or for a series of free brochures with tips on buying fine jewelry, contact Jewelers of America at 1185 Ave. of the Americas, 30th Fooor, New York, N.Y., 10036.