All pearls are measured in millimeters (mm), using a milligauge, which is an extremely accurate measuring instrument. Pearls are generally measured by their diameter, in most cases, the larger the pearl, the more rare and valuable it is.
Baroque pearls however, can be quite complicated to measure due to their irregular shape, they are usually measured from the widest point in their diameter or through the technique of sieving.
Pearls come in an array of shapes and as a natural product, each pearl is unique.
The most sought after and highly valued pearls are those that are perfectly spherical or round, only a very small percentage of cultured pearls are this shape.
A good way to tell if a pearl is perfectly round is by the way it rolls when placed on a flat and level surface, if it veers to one side, it is not round. There are six basic shapes defined at production. Shapes include:
- Round - A perfectly round sphere. A pearl is considered round when the variation in its diameter is less than 2.5%. For example, a pearl measuring 10mm can have up to 0.25mm variance.
- Near round - Very slightly imperfect shape, although the pearl may look perfectly spherical to the human eye, a pearl is considered near round when the variation in its diameter is more than 2.5%.
- Drop - A drop shaped pearl will always have a longer vertical axis than horizontal axis. Pearls that fall within this category are those in the shape of a teardrop, oval or egg shape.
- Button - Opposite to drop shaped pearls, the vertical axis of a button pearl will always be shorter than its horizontal axis.
- Baroque - A baroque pearl is considered asymmetrical or free formed and are extremely unique. When one side of a baroque pearl is round or symmetrical it is considered semi-baroque.
- Circle - A pearl is classified as a circle pearl when parallel grooves, bands or rings are present around the circumference of the pearl.
The colour of a pearl is primarily determined by the species of the oyster that it grows in, however the environment in which they are grown can also determine a pearl's colour and skin texture.
South Sea pearls derive their magnificent colour and exceptional lustre from the Pinctada maxima or Silver/Gold lipped oyster. The Pinctada maxima generally produces pearls in shades of White, Ivory, Silver and Blue with brilliant iridescent overtones of Pinks and Greens, however it can also produce the natural gold range of pearls with shades of Cream, Yellow, Champagne and Gold.
Tahitian pearls occur naturally in a remarkable range of colours from Aubergine, Peacock, Green, Grey, Blue to a deep Black all with various overtones. Tahitian pearls are produced by the Pinctada margaritifera or Black lipped oyster.
Akoya pearls produce naturally radiant shades of White, Cream and Ivory. A natural gift from the Pinctada fucata martensii oyster, Akoya pearls are famous for their magnificent, deep Pink overtones.
Freshwater pearls are a product from a variety of mussel species, primarily produced in the Hyriopsis cumingii or Sankaku mussel and the Hyriopsis schlegeli or Ikecho mussel, Freshwater pearls are naturally Cream, White, Peach, Pink, Lavender and Mauve. Freshwater pearls are also commonly colour treated or dyed.
The term lustre refers to the deep, luminous glow in a pearl that is visible to the human eye, it is the result of light reflecting both from the surface of the pearl and from light refracting within the layers of nacre. The quality and thickness of the nacre will determine how brilliant and iridescent a pearls lustre will be. Lustre is considered the single most distinguishing feature that pearls have over all other gems.
In terms of quality, the longer the pearl is in the oyster, the thicker the nacre and potentially the more luminous and rich the lustre. Nacre is the one factor that determines the longevity of a pearl. This is why nacre thickness can be considered the most important factor when considering the quality of a pearl. Quality of the nacre is also the one factor you can see clearly with your eye.
The surface of a pearl, generally referred to as the pearl's skin, is a build up of layers of nacre, the skin commonly contains imperfections a flawless surface is considered extremely rare. The surface of a pearl should be considered in two ways.
1. Blemishes - This includes imperfections such as spots, bumps, chips, scratches or wrinkle effects on a pearls surface.
2. Pearl grain - This refers to the structure of the pearl's skin, depending on how tight the structure is, this will determine how visible the pearl grain is. The tighter the structure, the grain will appear less evident.