X-ray of two Pearl Strands. The fake pearl strand at the top is made from plastic which x-rays can easily pass through. The real pearls are much denser an are not as easily penetrated. Real Pearls are accretions of nacre (calcium carbonate) secreted by certain oysters and mussels. The presence of a foreign body (such as a parasite or a grain of sand) within the shell triggers the secretion of nacre by the creature's mantle. Thin layers are deposited on the irritant particle and, over several years, a pearl is formed.