Although there is no universally recognized grading system for pearls, six main factors typically determine the value, quality and beauty of a pearl: Colour, Shape, Nacre, Lustre, Surface and Size.
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Pearl colour can range across the entire spectrum of the rainbow. When dealing with pearl quality and value, colour typically refers to the combination of two specific factors: body colour and overtone. The body colour of a pearl is its main, base colour. It can be categorized either as a warm-hue (purplish red to greenish-yellow) or cool-hue (reddish-purple to yellowish-green). Overtone is the overlying colour that appears to coat the surface of the pearl and can be seen within certain angles of light. A good quality pearl shows a deep and evenly spread overtone.
Despite general belief, not all pearls are round. Pearls can form in a variety of different shapes, including round, semi-round, drop, oval, half-rounded, semi-baroque and baroque shapes. Baroque shaped pearls are entirely asymmetrical and irregularly shaped. Typically, rounder pearls are considered to be of higher quality and greater value.
Nacre, also known as mother-of-pearl, is the iridescent material secreted by a mollusc to protect it from irritants, eventually creating a pearl. It also lines the inside surface of an oyster shell. It is made mostly of calcium carbonate and conchiolin. Generally speaking, pearls with thicker nacre are considered to be of higher quality.
Pearl lustre refers to its light-reflecting ability, typically described as brilliance or glow. Good quality pearls shine brightly, and sharply reflect light. If you were to look at a high quality pearl, you would be able to see your own reflection. The cloudier, chalkier or duller the pearl, the poorer the lustre, and the lower the quality of the pearl.
It is widely accepted that no pearl will ever be completely "flawless", but the highest quality pearls have nearly perfect, clean, blemish-free surfaces. When examining a pearl, check for spots, dents, cracks, discolorations, or any other imperfections. The surface of a pearl impacts its reflective power and therefore any bumps or flaws will typically lower its value.
To determine the size of a pearl, its diameter is measured in millimetres. If the pearl is not a perfect sphere, it is measured by the shortest diameter. It is important to remember that size alone can never determine the value of a pearl.