Technical Details of Pearls

Here are some physical characteristics of pearls:

The hardness: 2.5 - 4.5
S.G.: 2.70 (fresh-water up to 2.74)
Size: from microscopic to many centimeter diameter (rare)
Luster - typical pearly luster is termed "orient"
A variety of colors, depending upon the type of mollusk and the water composition (polluted water produces unusual colors!)
bodycolor: underlying color: white-yellow (cream), black
overtone: "float" (resembles a filmy lacquer): pink / green / blue
composition:
o ~ 86 % calcium carbonate (CaCO3)
o 2 - 4 % water
o ~ 10 % conchiolin (an organic binding agent)
Together, the conchiolin and CaCO3 are referred to as nacre

Nacre consists of a series of alternating layers of conchiolin and crystals of CaCO3. The CaCO3 is in the crystal form known as aragonite. The typical iridescence of the pearl is due to the series of nacre layers. This is referred to as 'orient'(iridescent effect due to overlapping nacreous plates)