T he way in which a mollusc creates a pearl is still layered in unanswered questions. Although enough is known about the process to manually replicate it and produce cultured pearls, there are still questions about an oyster’s nacre, also known as mother of pearl.
Nacre is a material secreted naturally by a mollusc to line the inside of its shell and protect its delicate flesh. This organic-inorganic composite material is composed of hexagonal-shaped platelets of aragonite (a form of calcium carbonate) arranged in extremely thin layers. These individual layers are separated by an organic webbing of elastic biopolymers or silk-like proteins that add surprising strength and resilience to the otherwise ‘chalky’ aragonite.
The characteristic iridescence of nacre is caused by the thickness of these layers of organic and inorganic compounds being similar to the wavelength of light. The pathways of light that enter the nacre and the light that is reflected outwards create different types of interference, resulting in shimmering multi-tonal colours when viewed from different angles.
But nacre isn’t just confined to mother of pearl and the lining of a mollusc’s shell. When an irritant enters a mollusc and gets trapped, this nacre is transferred onto the surface of the irritant as a defensive mechanism. Microscopic layers of nacre are added over time and eventually engulf the irritant, creating a pearl. As this is a natural process, the nacre is not laid down evenly but in fits and starts. The result is a delicate patchwork of ridges, lines and swirls that create each pearl’s own unique fingerprint.
Nacre is a significant factor in pearl pricing because it determines lustre and the flashes of colour that appear of a pearl’s surface. As a general rule, thick and evenly-layered nacre in a beautifully round shape generally results in a more valuable and more durable pearl. But don’t be fooled… there are some ‘pearls’ that aren’t made of nacre at all. The inner lining of some pearls is porcelaneous, resulting in a flame-like surface pattern. Take a look at ‘C is for… Conch Pearls’ to find out more.