Pearl or parasite enters a bivalve mollusk and settles inside the shell. The mollusk, irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant. This secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with perfectly round ones being comparatively rare. Typically, the build-up of a natural pearl consists of a brown central zone formed by columnar calcium carbonate (usually calcite, sometimes columnar aragonite) and a yellowish to white outer zone consisting of nacre (tabular aragonite). In a pearl cross-section
The Pearl of Death when caught the pearl is not found on him and he is released. Later, Holmes hears of an apparently motiveless murder. An elderly colonel is found with his back broken amid a pile of smashed china. Holmes takes an immediate interest in the case as the unusual method of killing is that of "The Hoxton Creeper" (Rondo Hatton), known to be Conover's right-hand man. Another murder occurs, of a little old lady, also surrounded by smashed china. Conover makes two attempts to kill Holmes, who surmises that Conover is desperately trying to recover the stolen pearl. After a third killing