Pearls showing these types were exhibited at a meeting of the Royal Society of London, June 8, 1887. That exhibit also contained a section of a west Australian pearl of curiously complex crystalline formation; instead of one central starting-point, it had more than a dozen scattered about, from which the crystalline prisms radiated in all directions.
Since the three superimposed layers of the shell are secreted by separate parts of the mantle, viz., the nacre by the general surface, the prismatic layer by the inner edge, and the epidermis by the outer edge, it follows that if a pearl in course of formation is moved from one of these distinctive portions of the palial organ to another, the nature of its laminæ changes. Thus, if a pearl formed on the broad surface of the mantle is moved in some way to the inner edge of that organ, it may be covered with a prismatic layer; if then moved to the outer edge it may receive a lamina of epidermis, and then by changing again to the broad surface of the mantle it receives further coats of nacre.
Pearls from common clam (Venus mercenaria) of eastern coast of America
Pearl “nuggets” from the Mississippi Valley
Wing pearls from the Mississippi Valley
Dog-tooth pearls from the Mississippi Valley