Periostracum — The Outer Organic Layer of a Mollusc Shell
Periostracum — The Outer Organic Layer of a Mollusc Shell
A conchiolin coating on the exterior of bivalve and gastropod shells, important to pearl culture protocols and to the identification of non-nacreous pearls
Periostracum is the outermost layer of a mollusc shell, a thin organic coating composed primarily of conchiolin — the same fibrous protein that holds together the platy aragonite of nacre in nacreous pearls. The periostracum is secreted by the mantle of the mollusc as the shell grows and serves as the first chemical and mechanical barrier between the shell and the seawater environment. For the gemmologist, the periostracum matters principally because of its role in the pearl-culture process and because its remnants on a pearl surface are diagnostic in some non-nacreous species.
Composition and function
The periostracum is a tough, leathery layer of cross-linked protein, typically brown to black in colour and millimetric to sub-millimetric in thickness. It is laid down by the periostracal groove at the edge of the mantle and protects the underlying calcium carbonate layers — the prismatic layer of calcite or aragonite, and beneath that the nacreous layer in nacre-producing species — from the dissolution that would otherwise occur in seawater that is not perfectly saturated in calcium carbonate. In freshwater bivalves living in soft, slightly acidic water, the periostracum is especially important, since the water is naturally undersaturated in calcium carbonate and the shell would otherwise erode.
The periostracum is also the source of much of the variation in shell colour. Many shells that look brown, black, or banded owe those colours to pigmented periostracum rather than to the underlying calcium carbonate; when the periostracum wears away, the lighter colour of the prismatic layer beneath is exposed.
Role in pearl culture
The standard pearl culture protocol involves grafting a small piece of mantle tissue from a donor mollusc into the gonad of a recipient mollusc, alongside a shell-bead nucleus. The donor tissue is the source of the cellular machinery that will form the pearl sac and deposit nacre around the bead. For the graft to produce viable nacre, the donor tissue must be drawn from the inner-mantle layer that secretes the nacreous layer, not from the outer-mantle layer that secretes the periostracum and prismatic layers.
Pearl-culture technicians therefore prepare donor tissue by removing the periostracum and the outer mantle from the donor shell and saving only the inner nacreous-secreting layer for grafting. A graft contaminated with periostracum-secreting tissue will produce a pearl with calcite or organic deposits on the surface, the so-called keshi coating that compromises lustre, or in the worst case will produce no usable pearl at all.
In non-nacreous pearls
Non-nacreous pearls — including the conch, melo, and pen pearls of the various non-nacreous host species — sometimes show remnants of periostracum on their surfaces, the result of the irregular pearl-formation process in molluscs that lack a true nacreous layer. The periostracum remnants are part of the diagnostic signature of these pearls under microscopic examination, alongside the prismatic structure and the lack of orient.
In the trade
For the working jeweller, the periostracum is a technical concept rather than a daily concern. The shells used for shell beads in pearl culture and for mother-of-pearl ornament have generally been worked to remove the periostracum entirely, exposing the underlying nacre or prismatic layer. The exception is in mineral-collector and natural-history specimen markets, where shells are often presented with the periostracum intact as part of the natural appearance of the species.