An introduction to pearls, the way they are created and the types of pearls used for jewelry. For more information see “The Types of Pearls Used for jewelry“.
Whilst the composition of pearls is primarily the mineral calcium carbonate, they are quite different in the way they have evolved compared with other mineral based gemstones. A pearl is a hard, rounded object produced by specific types of molluscs and oysters. Natural pearls are formed when a small foreign object, such as a parasite or grain of sand, becomes trapped inside the shell of an oyster. Then this is unable to be ejected, the mollusc secretes a substance called nacre to cove the foreign body in a succession of concentric layers. This creates a smooth, protective coating that, over time, becomes a pearl.
The nacre is secreted in very thin irregular layers, so that they overlap each other. This creates a surface that has microscopic ridges and is responsible for the way waves of light are reflected off the pearl to five the pearl its luster. The mineral composition of nacre is approx 92% calcium carbonate with the remaining 8% being organic matter and water.
Pearls obtain their iridescent qualities due to the layers of nacre being approx 0.5 micro metres – which is comparable to the wavelength of visible light. This enables the pearl to absorb and reflect the light.
The most usual – and also the most valuable pearl, is a spherical pearl that has good depth and covering of nacre. It can take as long as 2-5 years for a good quality pearl to fully evolve in an oyster. Pearl weight is measured in “grains”. Large pearls weigh in excess of one hundred grains and are called paragons, whereas small pearls weighing less than half a grain are called “seed pearls”. Any pearls weighing less than 1/25 of a grain are considered too small to be of economic value and called “dust pearls”.
Not all pearls are perfectly round and can occur in a wide variety of shapes, including oval, pear, baroque and others. The larger the pearl, the more it is likely to be irregular in shape. Some shapes – particularly if they are much larger than the usual spherical pearls – are valued very highly. The “La Regente” pearl is egg shaped and weighs 337 grains and is very highly prized. Another example is the Hope Pearl, weighing in at 3 ounces.
Most pearls these days are not naturally derived but cultured through human intervention. There are four main types of cultivated pearls:
Akoya pearls, were the first spherical cultured pearls, introduced early in the 20th century. These tend to be smaller than other pearls. Their luster is fair to excellent and blemishes vary from clean to moderately blemished. The Nacre also varies between thin and thick. Colors include White, Cream and some Yellow, Pink and Blue.
Freshwater Pearls: These are mainly cultivated in mussels in China, Japan and North America. They have thick nacre with fair to excellent luster and clean to moderately blemished surfaces. Colors are white, cream, gold, peach, pink and purple.
Tahitian Pearls: Cultivated in the black lipped oyster, these pearls have thick nacre, fair to excellent luster and clean to heavily blemished surfaces. The pearls are produced in blue, green, aubergine, grey and black.
South Sea Pearls. Australia currently supplies around 60% of the world’s South Sea Pearls. They are cultivated in either silver-lipped oysters or gold-lipped oysters and are the largest cultured pearls on the market. They have thick nacre, fair to excellent luster and clean to lightly blemished surfaces. Colors include White, Cream, Silver, Gold and Blue.
Cultured pearls that have had a large foreign body inserted into the mussel or oyster shell in order to increase the speed of the pearl’s development, will have only a thin layer of nacre. As a result the pearl will have a low level of luster, compared with a natural pearl or one that has been cultivated well.
See our Pearl Ring Selection.