A pearl’s iridescence is known as lustre and is absolutely unique to pearls, different to any other gemstone. When grading pearls, highly prized pearls will have strong depths of colour and a shiny lustre. Without lustre, pearls can be dull, opaque and lifeless.
The factors that determine a pearl’s lustre are highly complex, being determined physically by the way that light diffracts through the physical structure of the layered nacre of the pearl. The thickness of the nacre is caused by how long the pearl is left in the oyster. Generally, pearls with the greatest thickness of nacre will have the deepest lustre.
The colour of a pearl mainly reflects the type of oyster or mussel that the pearl was cultured in. However, a pearl’s complex overtones of colour can also be due to the temperature of the water, its cleanliness and even the plankton that the oyster feeds on. For example, colder water can introduce a light grey colour to a pearl and dirty water will create a brown overtone.
Unfortunately once the pearl is removed from the oyster, lustre doesn’t improve with age like a fine wine.
By selecting good quality pearls, and with careful cleaning, the lustrous qualities of a beautiful pearl can, however, be maintained over time. In a future article, we will look at how to take care of your pearls.IMAGE CREDIT: Copyright of Winterson