A pearl nucleus is moft often composed of freshwater mussel shell that has been ground into a spherical bead and polished. Other nuclei do exist that have been created marine shell mother-of-pearl and bironite.
Nacre Deposition Will Occur On Any Solid Object
An oyster's pearl sac will secrete nacre on nearly any solid object. This has led to countless attempts to nucleate oysters with material other than oyster shell. Success has been limited, however, and oyster shell is still the main staple of the pearl farmer as it has been since the early 1900's. Some success has been reported from a new nucleus composed of bironite, but is not widely used.
It Takes A Special Material To Create The Optimal Nucleus
There are several factors that must be in place for the successful culturing of pearls on a large scale. These are the reasons nuclei of non-standard composition has been so quickly rejected in the past:
- The density of the nucleus must exactly match, or be extremely close to the density of the host mussel. This density is measured to approximately 2.8g/cc.
- In order for the pearl to expand and contract in different environments, the nucleus must expand and contract in a compatible fashion. This is known as the thermal coefficient of expansion.
- The nuclei must also resist cracking, hold a high shine, and remain stable over long periods of time.
The Best Material Comes From The United States
The materials that best fits these criteria are the shells found in the Mississippi River Basin. Many of the mussels from this area have the added attribute of a thick shell, especially in the joint where the bivalve connects. This thick shell enables harvesters to create large nuclei to be used in culturing larger pearls.
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