Pearls are organic gemstones that are formed by shelled molluscs mainly bivalved oysters and mussels. Pearls are made up of nacre (mother-of-pearl) which is mostly aragonite (calcium carbonate) and conchiolin (complex proteins that form mollusc shells). The aragonite microcrystals build up around an irritant. The name "pearl" is said to have originated from the Middle English word "perle", which in turn came from the Latin word "perna", meaning "leg", thought to be due to the ham-leg shape of the bivalve mollusc.
Natural pearls are extremely rare, incredibly expensive and typically small. Therefore people have developed ways to culture pearls, so that these beautiful gemstones can be enjoyed by many. In cultured pearls, some tissue or a mother-of-pearl bead is introduced into the mollusc shell. If successful, this process induces the animal to form a "pearl sac" whose cells secrete a layer of brownish protein called conchiolin over the irritant. This is followed by the secretion of numerous mineral layers of nacre composed of calcium carbonate in thin overlapping plates. Pearls can come from fresh or seawater molluscs. The beauty of pearls is that they can be plucked from the shell naturally beautiful, fully-formed and displaying perfect lustre with no need to be cut or polished.
The oldest recorded reference to pearls in history is the 7,500 year-old "Umm Al Quwain Pearl", which was found in a grave in a place that is now known as the UAE. Before pearls were cultured by man, they were harvested from the Persian Gulf, Sri Lankan waters, fresh water sources in China and the rivers of Europe. Later, Christopher Columbus discovered pearls in South America. When these natural pearls were almost depleted in the early twentieth century, the Chinese and Japanese began to culture pearls and the rest is history. Nowadays, pearls remain a treasured adornment and are cultured all around the world.
Identifying Pearl 
Pearls can be identified by their pearly lustre and when rubbed gently against the teeth, a slightly rough surface texture can be detected, whereas imitation pearls feel smooth. The surface appearance of natural and cultured pearls is the same, but the density of cultured pearls is usually higher, at approximately 2.73 for most. The only definite way of distinguishing between natural and cultured pearls is to examine their internal structure. Experts use endoscopes to do this through the drill holes of pearls. Natural pearls have concentric internal layers.
Origin and Gemstone Sources 
Pearls are found and cultured in waters all over the world. Natural sea pearls are found in Australia, Japan, Central America, the Persian Gulf, the Gulf of Manaar (between India and Sri Lanka), the coast of Madagascar, Burma (Myanmar), the Philippines, the South Pacific Islands (including Tahiti and Fiji) and South America.
Natural river pearls are found in Asia, Europe and North America.
Cultured seawater pearl sources include Southeast Asia (such as Indonesia and the Philippines), Australia, China, French Polynesia, Japan, South Pacific Islands (including Tahiti and Fiji) and the Philippines.
Cultured freshwater pearl sources include China and Japan.
Pearl Colour
Pearls are available in a variety of colours. Colours of pearls include the following: White, pink, silver, cream, golden, green, blue and black. Some pearls exhibit iridescence, which is known as orient.
Pearl Clarity and Lustre
The lustre of pearls depends on the quality of the nacre. Pearls should have the characteristic shiny pearly lustre and their surface should show sharp and bright reflections. The surfaces of good quality pearls are smooth and blemish-free with a suitable layer of nacre to increase durability.
Pearl Cut and Shape
Pearls can be round, oval, pear-shaped or misshapen (baroque pearls). However, the best materials are regularly shaped. The most valuable shapes are symmetrical spheres or symmetrical drops. Pearl sizes depend on the type of mollusc that they form in and they typically range from 2 mm to 16 mm in diameter.
Pearl Treatment
Pearls are often bleached to lighten and enhance their colour. In this way, a uniform colour can be achieved for beaded necklaces.