When a mollusk senses an irritant within its body it secretes layers of semi-opaque calcium carbonate called nacre (NAY-ker) to coat the irritant. This substance builds up over time and becomes a pearl.

The thicker the nacre, the more lustrous the pearl. Though there are exceptions to that rule, the amount of nacre determines the reflective quality of the pearl's surface. And luster is the most important factor determining the quality of a pearl. It is especially important that the surface be without blemish, because luster is more often seen on a smooth surface without ridges that deflect the light.

A low-luster pearl is dim and monochromatic, but a higher grade of pearl with thicker nacre produces a resilient pearl with far few surface blemishes. This creates a sharp, high-contrast surface that is almost mirror-like in its reflective qualities.

"Very high", "high", "medium", "low", and "very low" are the five categories used in the industry to grade the luster of any pearl.

Grading Chart - Nacre Thickness (Based On GIA Pearl Grading System)		Value %	Grade
Very Thick	0.5mm thick on all pearls	100%	AAA
Thick	0.5mm thick on most pearls	90 to 95%	AA
Meduim	0.35mm and 0.5mm on most pearls	80 to 85%	A+
Thin	0.25mm and 0.35mm on most pearls	60 to 75%	A
Very Thin	0.25 or less on mostpearls	25 to 55%

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Courtesy of Mondera