Not A Grain Of Sand
To best answer this question it is important to point out how natural pearls do not form. They do not form from a grain of sand. The grain of sand theory is a widely accepted urban legend. The generally accepted theory of natural pearl growth is the Parasitic Invasion Theory. Parasitic Invasion
The theory of parasitic invasion describing a boring worm, or a boring parasite that burrows through the outer layer (periostracum) of a mollusk and breaks through the inner layer (hypostracum) and comes into contact with the mantle muscle of the mollusk. Mantle Tissue
The mantle organ is responsible for the deposition of the calcium compound which lines the interior of a mollusk's shell. The cells responsible for this are known as epithelial cells. Pearl Sac
When the parasite becomes lodged in the mantle, the epithelial cells begin to grow into a sac and surround the parasite. The lining of the sac has the same properties as the outer edge of the mantle organ; the lining excretes a calcium carbonate and aragonite compound known as nacre. The Natural Pearl
As the pearl sac deposits nacre around the parasite a pearl is born. A pear is millions of layers of nacre piled one on top of another in a hexagonical, brick-like structure.