In order to understand the structure of the pearl it is necessary to understand the anatomy of the mollusc that produces it.
The main parts of the mollusc are the shell, the mantle and the mollusc's organs.
The shell has an inner section made of calcium carbonate in its two mineral forms of aragonite and calcite.
These minerals may consist of the same elements of carbon, calcium and oxygen but the arrangement of the atoms are different.
The two forms can be present in the same shell.
The outer face of the shell is often protected by a brown, organic, horny shaped covering called conchiolin.
This material is the substance which binds together the minerals aragonite and calcite and is evenly dispersed between the crystals of carbon carbonate.
The mantle is genetically programmed to produce both the calcium carbonate and the conchiolin for the shell and to produce pearls.
Pearls are not made of the same consistent material.
Around 90% of a pearl consists of crystals of calcium carbonate, 5% of conchiolin and the rest of water.
They also contain organic proteins which are the source of its colours.
These proportions will vary depending on the species of the shellfish.
For example, there are fragile black pearls from the Philippines which contain over 15% of water and gradually disintegrate when removed from the sea.
Most pearls are composed of aragonite which crystallises in two ways.
They crystallise as flat crystals with a hexagonal shape and as needle-shaped prisms.
Light is split as it passes through these individually shaped crystals producing different illuminating effects.
It is only the manner in which the outer layer of a pearl reacts to light that differentiates nacreous from non-nacreous or 'porcellaneous' pearls.
Studies conducted in the growth of pearls hypothesise the importance of conchiolin in determining the mineral structure of the pearl.
The conchiolin supports the crystals of calcium carbonate and also binds molecules of water strongly together to form the pearl.
This organic material may be responsible for the solidity of the pearl and the elasticity that cushions shock.
Nacreous pearls grow in layers that form tiers with the conchiolin as the cement binding the platelets forming these tiers.
The centre of the nacreous pearl often consists of a substantial concentration of organic material together with columns of calcite crystals.
This is followed by concentric deposits of polygonal aragonite blocks.
These platelets must be arranged in a regular way without gaps or defects to produce the most beautiful nacre possible.
As light passes through the upper layers of aragonite, it is reflected out again producing the mercurial effects that create the rare and beautiful iridescence of nacre.
Just as pearls composed with flat hexagonal crystals of aragonite are nacreous, those composed of needle-shaped prismatic crystals of aragonite are called porcellaneous.
They are the same from centre to surface with elongated fibrous crystals radiating from the centre.
This structure makes them hard and solid.
An example of a porcellaneous pearl is the Melo or the pink pearl of the queen conch of the Caribbean.
Depending on the species of the mollusc, the elements have the shape of ultra-fine fibres which form bundles that are arranged in a crisscross pattern.
These pearls produce different light effects from nacre.
Though some may appear bright and some appear dull, light actually propagates through elongated prisms of aragonite creating the most splendid and exquisite flame-like effects.
The idea that only 'nacreous' pearls should be called pearls is nonsense.
All of the 'creative concretions' produced by molluscs with shells should be called pearls.
They are formed in the same way.
They are composed of the same organic material.
Only the external appearance and the way in which they react to light are different and the magnificent light effects called 'flames' are just as spectacular as the shimmer of nacre.