The term 'SOURCE OF NATURAL PEARLS' can be answered with the 6-letter word 'OYSTER'. Oysters are mollusks that produce pearls as a result of an irritant, such as a grain of sand, entering their shell. Natural pearls are formed when the OYSTER secretes a substance called nacre to coat the irritant and form a pearl. Oysters are the primary source of natural pearls, although other mollusks, such as mussels, can also produce them.