'a piece of s---'

'The pearl,' says Bari, 'is the opposite of the diamond, which is completely mineral, the hardest thing known in nature. The pearl is soft, the product of a living organism. Its mystique lies in the mystery of its creation. The ancient Greeks believed pearls were formed from the tears of the gods. In Christian times the birth of the pearl was seen as a kind of immaculate conception, symbolising purity and closely associated with the Virgin Mary.' 
In fact, as he goes on to explain, the pearl is just 'a piece of s---': generally a tapeworm larva that gets stuck inside the shell of a mollusc. The shellfish reacts to protect itself, building up layers of shell tissue, called nacre – or mother of pearl – around the invading particle.
Mark Hudson, "Pearls: the tears of the gods." The Telegraph 17 September 2013.

Photo of pearls forming in the shell from the American Museum of Natural History.