| The first thing about a fine pearl
that you notice is its lustre or brilliance. Lustre is not a superficial
"shine" such as you see in imitation pearls, but an intense
brightness that results from rays of light travelling through the
numerous layers of nacre and being reflected back from within the
pearl. A pearl with high lustre is bright and extremely reflective.
This quality is important as any colour can be beautiful but exceptional
lustre is what will set a pearl apart. Lustre and orient are important
not only because they affect the pearl's beauty because they are a
visible indicator of nacre thickness.
Lustre is evaluated on the sharpness or brightness of the
reflection of light from the surface. This depends on the quality
and thickness of the nacre produced by the oyster. The thicker the
nacre (and smaller and more transparent the microscopic crystals
comprising it), the better the lustre. When the layers of nacre
have crystallized properly and each layer is well aligned the result
is an exquisite, intensely lustrous pearl. Light is absorbed and
reflected and refracted giving a pearl its brilliance and very occasionally
a color halo.
Orient is defined as iridescence or the play of rainbow
colours across the surface of a pearl. It differs from lustre, although
the two terms are sometimes used interchangeably. The term probably
originated from the fact that orient, or iridescence, is best seen
by rotating a pearl to align it in different directions. Another
possibility is that the term stems from the original Latin meaning
of oriens, "the rising of the sun".
The same criteria that determine the intensity of lustre also influence
the quality of orient. The predominant factors are the thickness
of the nacre and the regularity of the component nacreous crystals.
Multiple layers of consistently thin nacre, composed of perfectly
formed nacreous crystals, produce the finest orient. It is most
commonly visible on irregularly shaped pearls having uneven surfaces
where the nacre "collects in deep pools" - that promote
iridescence.
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