Jewellery Catalogue   PEARLS are an organic substance classed
alongside precious gems. When pearls first arrived in Europe
from the Middle East, they were more expensive than diamonds.
Colour and lustre are important for assessing the value of a
pearl.  Pearls are formed when a grain of sand or irritant is
present within the mollusc. In cultured pearls this irritant is
introduced manually, it is then surrounded by layers of nacre
until a pearl is formed. The thicker the nacre, the more
lustrous the pearl becomes.  There is a wide colour range
including white, black, grey, pink, and yellow.  Natural pearls
are relatively rare, for environmental reasons and because
cultured pearls are so common. Most natural pearls come from the
oyster but conch and fresh water mussels also produce natural
pearls. The oriental pearls have a different lustre and sheen
from freshwater pearls, colour and lustre are important
qualities in pearls.The more regular the shape of the pearl the
more valuable it is.Blister pearls are form