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June Birthstone - Pearl
Physical and Chemical Properties
Chemical composition
Pearl is unique among the birthstones inasmuch as it is of organic origin. All
other birthstones are minerals, inorganic solid substances with crystalline
structures and fixed chemical compositions that vary only within rigid limits.
Pearl are made up of little overlapping platelets of the mineral aragonite, a
calcium carbonate that crystallizes in the orthorhombic system. In cross section
a pearl will appear to have concentric, smooth layers, but magnification will
show these layers have an imbricate (brick wall-like) structure. These tiny
plates are held together by an organic cementing agent called conchiolin. The
pearl derives its iridescence from the diffraction and interference of white
light that is caused by the tiny overlapping platelets of calcium carbonate. The
iridescence or orient of the pearl is a function of the numbers and thickness of
these platelets. Mother of pearl or nacre forms on the inner walls or inner
surfaces of the mollusk shell. Mother of pearl differs from pearl inasmuch as it
is part of the mollusk shell whereas the pearl has become a separate entity from
the shell.
Colors
In America, the most commonly bought pearls are white with pink tones. But pearls
may also be tan, yellow, brown, gray, or black with a purple or green luster.
The color of this june birthstone depends on the color and chemicals found in the
shell of the mollusk.Most pearls are spherical, but irregularly shaped specimens
are called "baroque."
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