The formation of natural pearls

Although  the story of the formation of natural pearl is really like a myth, it still be discovered by human beings.
Natural pearls are 100% pearl or nacre. The formation of a pearl is thought to be that natural pearls form under a set of accidental conditions when a microscopic intruder or grain of sand enters an oyster (mollusk) and settles inside the shell. In a defensive response to the irritant, the mollusk secretes nacre as a smooth, protective coating. Nacre is a combination of crystalline and organic substances that form the iridescent mother-of-pearl lining of mollusk shells. The nacre is secreted by the epithelial cells of the mantle tissue in certain species of mollusk. From the outside in, the pearl sac (the cells) secrete nacre and conchiolin unto the irritant and the pearl starts to grow. The pearl sac expands through cell division as the pearl grows.
In this illustration a "free" pearl is formed. When the irritant attaches itself to the shell, then there will be no formation of a pearl sac and the epithelium cells just deposite nacre and conchiolin on the irritant and the inner shell, creating a blister. For pearls to form in nature it can take may years of near-perfect conditions for them to make a significant gain in size.
Since natural pearls take longer to develop, it can take two to five years for a quality pearl to fully develop in the oyster. However, natural pearls continued to have a regular market until the nineteen fifties. After this, until today, they have had a small but fine market for themselves.