(a)Pearl is actually an 'inside out' tiny shell secreted by a bivalve like ealm and Oyster in response to an injury, parasite or foreign irritating substrance like sand and grain.
(b)The shell is opened and a fine silica or other particle is introduced between the mantle and the shell.
(c)The mantle starts secreting nacre over the foreign particle.
(d)In few years it grow in a pearl. Pearl formation was first introduced in Japan by Mikimoto.