Pearls: Nature’s Rare Gift

All of the members of the class Bivalvia, belonging to the phylum molluscs, live in water. Their solid shell consists of two valves, which are opened by using the adductor muscle. Occasionally, a grain of sand or a parasite may enter between the shell and the mantle flap, causing the glands of the mantle flap to encase them with nacre. More and more layers are added, making the noble pearl bigger.

Pearl formation

Growing pearls is a protective mechanism in the molluscs.
When a parasite enters the shell,
it is surrounded by several layers of nacre in order to isolate it. It might take as long as a decade to form a pearl the size of a pea.

The parasite enters
the shell.

The mollusc produces nacre to surround it.

Cultured or farmed pearls are produced by inserting a ‘nucleus’, such as
a small bead, inside
the mollusc to stimulate pearl formation.
Then, the molluscs are
put into nets and placed back into the water where pearl formation begins.

Pearl hunting was the main means of obtaining pearls, as well as the main occupation of natives in coastal regions, until the appearance of pearl cultivation. However even today, some still practise this craft. Pearl hunters dive to depths ranging from 7 to 22 m, and stay down
for as long as they can hold their breath. The most experienced hunters usually manage to stay under for 70–90 seconds, but one
case in Tahiti reported a pearl hunter that was submerged for more than 2 minutes! Of course, nowadays it can be useful for these craftsmen to make use of modern diving equipment.

Pearls are considered valuable gems and have been used for making je­wellery since ancient times.
They vary in shape, size and colour.

Tahitian black pearls

Shell

The inner surface of some types of molluscs consists
of three layers, he innermost of these being nacre
or mother of pearl.