One hears sometimes that it is more difficult to assess the value of a pearl than that of a gem, especially since pearl quality is subject to a large number of factors. Experts base their opinion, among others, on the following objective and/or visual observations:

• Colour
  Colour is determined by the shell. The pearl industry distinguishes between the following colour groups: pink, silver, cream, gold, green, blue and black. This discrimination by colour is obviously rather subjective, being evidently determined in daylight conditions. The popularity of certain colours differs everywhere, being often linked with skin colour. Silver white and pink pearls are a perfect match for pale skin colours, whereas more cream-coloured pearls are ideal for dark skins.
  
  
• Dimensions
  Pearls are measured by gradations of half a millimetre, e.g. 6.0 x 6.5 mm. Larger calibres, being hard to cultivate, are more expensive. A sufficiently thick mother-of-pearl rim around the core guarantees brilliance quality and pearl durability.
  
  
• Iridescence and brilliance
  Iridescence is the rainbow glow created by the quality, thickness and regularity of the mother-of-pearl layers. These factors confer a magical brilliance, apparently radiating outwards from the core, to the pearl. Lustre or brilliance is more a matter of reflection power. The deeper the lustre, the more precious the pearl. Pearls with a cloud-like or milky brilliance are rated low.
  
  
• Surface
  Layer regularity, layer roughness or smoothness and structural impurities determine this essential criterion. Non-marked pearls are always chosen. Purity means the absence of surface defects, i.e. small marks or deformities during the growth process.
  
  
• Shape
  The rounder, the more perfect. However there is often a preference for pear-shaped or tear-like, oval, flat, semi-round or baroque pearls. Sometimes these are natural shapes, sometimes they result from certain implants.
  However they always constitute a source of inspiration for jewel designers.