Pearls are classified by their type, size, shape, lustre, nacre finish and colour.
A natural pearl is a pearl produced by a mollusc as a natures way of reducing an irritant that has penetrated through the shell or body of the mollusc. All bivalve molluscs can produce pearls, however only a few varieties produce what we call gem quality pearls.
The Akoya oyster produces what is called the "Japanese" cultured pearl. Though the culturing process was not invented by Mikimoto, he was the Barnam and Bailey of the pearling industry and created the market for the cultured pearl. The sizes range from the tiny seed pearls to up to 10mm. in diameter.
South Sea Pearl:
The oyster variety Pintarda Maxima produces the largest pearls, in an array of hues which are known as "South Sea Pearls". Australia is famed for producing some of the world's finest South Sea Pearls. The pearls generally range from 8mm to about 17mm in diameter, though they have been known to reach up to 20mm.
Fresh water pearls are produced by mussels. Originally looked upon as the poorer cousin to salt water pearls, the techniques being used and developed in the mussel culturing process have made the the difference very difficult to detect. Fresh water pearls come in a variety of shapes, colours and sizes.
The last cycle of the culturing process is the making of the mabe or blister pearl. This is achieved by gluing a dome, tear drop or other shape to the inside of the shell of the mollusc and allowing the oyster to coat the implanted shape with nacre.
The Black Pearl:
The queen of the sea. The black pearl is produced by a special variety of a large oyster known as the "Black Lipped Oyster". Due to its rarity, French Polynesia and the Cook Islands have jealously guarded these oysters. The sizes achieved are similar to the Pintarda Maxima or "South Sea Oyster". Black pearls are classified in the same way as other pearls.
No other issue in the pearling world is more hotly debated than the classification of the Keshi pearl. Keshi pearls are produced as a by-product of the culturing process. They are produced by the oyster rejecting the implanted nucleus and organicaly producing a pearl of its own. Keshi pearls are ver irregular in shape.
In Pearls, size does matter. It is one of the fundamental criteria of how a pearl is classified and valued.
Uniform round: The Pearl should be extremely uniform in shape and roundness.
Semi round: The pearl should be only slightly off round.
Semi baroque: A pearl with a distinct off-round to irregular shape which is easily recognisable to the eye.
Baroque: A pearl with a highly irregular shape. It may elongated and show what is known as a "tail".
Teardrop: A pearl which is in the shape of a tear drop.
Button: A pearl with a flattened, button like profile.
Luster is dependant on the quality of the oyster, how well it has created the nacre of the pearl and how long it has been given to do so. The "orient" of the pearl, which gives it it's luster is due to the refraction of light on the nacre and gives the sheen and hue which is so sought after in pearls.
Pearls come in a variety of colours. Some occur naturally while others are treated. White pearls occur in various shades of silver and cream. Golden pearls fetch a premium price for their warm hue. Black and silver pearls are created by the black lip oyster and are often wound with a secondary overtone of aubergine or peacock green.