Natural pearls are almost 100% calcium carbonate and conchiolin. Natural pearls are thought to form under a set of accidental conditions dictated by a microscopic intruder or parasite that enters a bivalve mollusk and set up shop inside the shell. Irritated by the intruder, said mollusk forms a pearl sac of external mantle tissue cells and starts secreting calcium carbonate and conchiolin to cover the irritant. Continuous secretion produces a pearl of which perfectly round natural pearls are comparatively rare.
Typically, the constituents of a natural pearl, consists of a brown core formed by columnar calcium carbonate (usually calcite, sometimes columnar aragonite) and a yellowish to white outer, consisting of nacre (tabular aragonite). The presence of columnar calcium carbonate, rich in organic material, indicates juvenile mantle tissue that formed during the early stage of pearl development. Displaced living cells with a well-defined task may continue to perform their function in their new location, often resulting in a cyst. Such displacement may occur via an injury. The fragile rim of the shell is exposed and typically prone to damage and injury.
Predators such as crabs and parasites such as worm larvae cause traumatic attacks and resulting in injuries causing the disconnection of external mantle tissue cells from their layer. Embedded in the conjunctive tissue of the mantle, these cells may survive and form a small pocket in which they continue to secrete their natural product: calcium carbonate. The pocket is called a pearl sack, and grows with time by cell division; in this way the pearl grows also.
The juvenile mantle tissue cells, according to their stage of growth, produce columnar calcium carbonate, which is secreted from the inner surface of the pearl sack. With ongoing time the external mantle cells of the pearl sack proceed to the formation of tabular aragonite. When the transition to nacre secretion occurs, the brown pebble becomes covered with a nacreous coating. Throughout its life, the shell itself grows and the pearl sack seems to travel into the shell, even though it actually stays in its original relative position within the mantle tissue. A single pearl takes a minimum of two years to form, upon which the shell might be found by a lucky pearl fisher.