Pearls being made from calcium carbonate are one of the softest gemstones. They have a very fine surface made from a material called nacre which is secreted by the oyster to coat an irritation within its shell such as a grain of sand. Nacre is very soft (2.5-4.5 on the Mohs scale) and easily scratched so pearls should always be kept separately from other jewellery to minimize scratching.
They should be kept clean and dust free by gently rubbing them with a soft cloth and should never be immersed in an ultrasonic cleaner. Never spray your neck with perfume or put foundation on your neck if you are planning to wear your pearls. Pearls are porous and either of these products will damage their lustre.
Never shower or swim in your pearls, as this can also rot the silk on which the pearls are threaded. Even the best looked after pearls will need to be checked professionally every couple of years if you are wearing them regularly, as they may have stretched over time and may need to be restrung. Once the thread has stretched it creates gaps between the pearls or the knots if the necklace has been threaded with a knot in between each pearl. Once the pearls are able to move up and down on the silk, the silk can be weakened and in time may break.