Pearls are rounded concretions composed of calcium carbonate – specifically aragonite and calcite – and organic matrix, secreted from particular marine and freshwater molluscs. There are two kinds: nacreous and non-nacreous.
The surface of a nacreous pearl is formed from thin layers of nacre, or mother-of- pearl, that are deposited as microscopic, tile-like crystals. The iridescence produced by a pearl’s nacre is caused by the diffraction of white light as it interacts with the arrangement of these aragonite crystals.
Non-nacreous pearls may be produced by the pink or queen conch (Strombus gigas), the giant clam (Tridacna gigas) and several species of edible oyster. They do not display an iridescent surface. Instead, these pearls commonly have a porcelain-like appearance due to their calcite or mixed calcitic-aragonitic composition. They are still admired for their pale colours and lustrous surfaces and are used for both jewellery and art.
The quality factors of a pearl may be broken down to: shape and size, body colour, iridescence, lustre and the presence and intensity of the ‘orient’ – a dramatic, multicoloured iridescent sheen.
The body colour of a pearl is thought to be caused by organic pigments present in the matrix. The most familiar colours of nacreous pearls are white and cream. They may also be black, grey and silver. Pinctada maxima oysters tend to produce silver or champagne pearls, Pinctada margaritifera produce black pearls, and Pteria penguin pinkish pearls with the mabe´shape.
The iridescence of the nacre can create overtones of pink, green, purple or blue.
While the perfectly spherical pearl is highly prized, each shape has its own beauty and suits particular jeweller settings. These shapes include button, oval, drop and baroque, as well as mabé, which is a pearl that is flattened on one side.
Keshi pearls are a little surprise of nature and highly sought after due to their exquisite lustre and unique freeform shapes. These small baroque pearls are typically formed as by-products of pearl cultivation.
Natural pearls are produced without any human intervention and are extremely rare, with almost all available pearls in the market today being cultured by man. Culturing involves ‘seeding’ live molluscs with a small shell bead, upon which the creature forms a nucleus and begins to secrete its nacre.
Commercial production of cultured pearls began in the early 20th Century and has changed a great deal since. Whilst there is a level of control over the size and shape of the pearls cultivated, the mollusc is still very much in charge of the process, alongside the forces of nature.
A gem that does not require further polishing or faceting to enhance its beauty, a pearl is simply perfection in its natural form. Their delicate iridescent sheen is captivating and can be enhanced by the presence of other gems or simply dazzle on its own. Beautiful as single spherical shapes, baroque forms or long strands, pearls are a classic jewellery staple.
More reading:
Organic gems part 1: Amber