Pearls are formed within the soft tissue of mollusks and other shelled animals as a defense mechanism against irritants. They are composed primarily of calcium carbonate in the form of aragonite or calcite deposited in concentric layers. Pearls have been highly valued for their beauty and rarity for centuries. While softer than other gemstones, pearls are one of the few that do not require cutting or polishing to enhance their natural beauty.

2.  A pearl is a hard object produced within the soft tissue (specifically
the mantle) of a living shelled mollusk or another animal, such as
a conulariid.
 Just like the shell of a mollusk, a pearl is composed of calcium
carbonate (mainly aragonite or a mixture of aragonite and calcite)in
minute crystalline form, which has been deposited in concentric
layers.
 The ideal pearl is perfectly round and smooth, but many other
shapes, known as baroque pearls, can occur.
 The finest quality natural pearls have been highly valued
as gemstones and objects of beauty for many centuries. Because of
this, pearl has become a metaphor for something rare, fine,
admirable and valuable.

3.  Formed by organic activities.
 Pearl is among the highly prized gems despite its
inferior hardness (H=2.5-3.5) and chemically
reactive nature.
 It is one of the earliest gems known to human
being.
 It is only gem that does not require lapidety
treatment or cutting, polishing or modification.
 If at all any craftsmanship required, it is the
perforation for stringing.

4.  Pearl is made up of concentric layers of organic
material called conchiolin (C32H48O11) and platelets of
aragonite contain 2% of SrCO3. Strontium carbonate
 Conchiolin acts as cementing material between
platelets of aragonite.
 Conchiolin’s concentration is higher between two
layers.
 Individual layers are known as nacre or mother-of-
pearl (m.o.p).

5.  Nacre layers are concentrically disposed around
nucleus.
 The proportion of aragonite is about 82-86% and
that of conchiolin is 10-14%. Apart from these two
substances, pearl comprises 2-4% water.

6.  Only a few molluscan species are capable of
producing pearls. Most common species belong to
genus “pintada”. Which lives in salt water,
amongst them, “oysters” are common.
 The growth of pearl starts from secretion of nacre
from mantle around any irritant that enters the
oyster.
 Such irritant occupy position between the mantle
and soft part of animal.

7.  The irritant may be some minute particle like an
insect or worm, sand particle or even an injury in
the mantle.
 The nacreous “sac” or pouch gradually separates
from the mantle to form a “free pearl”.
 Often the pearls may remain attached to host
shell.

8.  Specific gravity=2.61-2.78.
 The refractive index does not have importance in
pearls.
 Measurement of RI is difficult as it does not have
flat surface, and it is a mixture of cryptocrystalline
and amorphous material and its birefringence is
extremely high.

9.  Most important property of pearl is its appearing
“pearly” lustre or sheen.
 It is an optical property produced by diffraction of
light from thin overlapping seams of nacre.
 Delicate iridescent reflection adds to the beauty of
pearls.

10.  The kind of lustre is named after this gem, the
pearly lustre.
 Some oyster shells display iridescent where
nacreous layers (m.o.p) are uniformly thin.
 On aging or prolonged use, pearls lose the sheen
due to loss of water content and reaction with
body oils/acids and acidic content of tap water and
also cosmetics.

11.  Colour = milky white with tinge of yellow,
gun metal gray and even black.
 Sometimes pearls are stained to rose pink and
other colour and treated with dilute silver nitrate
(AgNO3) for blacking pearls are measured in terms
of grains.

12.  1 grain = 0.25 carat or 50 mg.
 Terms used for describing pearls are, natural
pearls, cultural pearls and Imitation pearls.

13.  pearls grown around an irritant or a foreign body
that accidentally enters the shell of a mussel
(mollusc) in nature.

14.  pearls grown in mussels (usually cultured) with
deliberately introduced large nucleus as irritant
usually bead cut from freshwater shell.

15.  Beads of glass and other substances coated with
“pearl essence” made up of fish scale and other
material.

16.  Although pearl oysters are wide spread globally,
not much are recorded from Indian territory.
However, the region between India and Sri Lanka
known for pearl oysters.