Pearls are the only gems made by living creatures. An oyster or mussel creates a pearl by coating the irritant inserted into its shell with thousands of layers of a milky substance called nacre. Those layers give pearls their lustrous, almost translucent appearance. The layers of nacre are extremely thin so pearls need gentle handling. Everything written in jewelry care about gemstones being affected by chemicals applies to pearls to an even greater extent because of their porous surface. Vinegar and ammonia (contained in some jewelry cleaners) are especially destructive and will eat into a pearl's surface.
Minute amounts of water trapped within the layers of nacre give pearls their translucent depth. Pearls will lose their beauty if kept in a too-dry environment, so do not store them in an airtight plastic bag or in a safe deposit box.
Even though pearls need moisture, strands of pearls should never be immersed in water. The outer layers would be fine, but if water gets into the inside of the pearls through the hole drilled in it, they may not dry out and that could cause damage. It's best to wipe your pearls gently with a damp cotton cloth if they need cleaning.
Pearls are traditionally strung on silk thread, with a knot between each pearl to keep them from rubbing against each other and damaging the nacre. It also keeps the pearls from moving on the thread so the drilled hole in each pearl does not get larger. An additional benefit of knotting between pearls is that if the strand accidentally breaks, only one or two pearls will drop and the rest of the strand will stay intact. As with gemstones strung on silk, the thread will be damaged if it gets soaking wet.
The best way to maintain your pearls' luster is to wear them. After you take them off, follow the jewelry care instructions written about in how to clean jewelry. When the pearls are dry, wrap loosely in jewelers' anti-tarnish tissue and store in a gift box or soft pouch.