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Question

Pearl is a hard object produced within the soft tissues of a mollusk. Which one of the following is the main constituent of pearl?

This question was previously asked in
CDS I 2017 General Knowledge Previous Year Paper (05-Feb-2017)
The correct answer is

Calcium carbonate

Understanding the Composition of Pearls

Pearls are fascinating natural treasures formed within the soft tissues of certain mollusks, such as oysters and mussels. They are created layer by layer as the mollusk secretes a substance called nacre, also known as mother-of-pearl, around an irritant inside its shell.

The question asks about the main constituent of a pearl. Let's look at the options provided and determine which substance makes up the largest part of a pearl's composition.

Main Constituent of Pearl Explained

Pearls are primarily composed of calcium carbonate in a crystalline form. This calcium carbonate is laid down in tiny, overlapping layers. The specific crystalline form found in pearls is mainly aragonite, although calcite can also be present. These crystals are cemented together by a protein known as conchiolin. The arrangement of these layers and the interaction of light with them give pearls their characteristic luster and iridescence.

Analyzing the Options

  • Calcium carbonate: As discussed, calcium carbonate is the primary building block of pearls, forming over 90% of their composition.
  • Calcium oxide: Calcium oxide (\(\text{CaO}\)), also known as quicklime, is a different chemical compound and is not the main constituent of pearls. It is formed from calcium carbonate but through a high-temperature process (calcination) that doesn't occur in mollusk tissues.
  • Calcium nitrate: Calcium nitrate (\(\text{Ca(NO}_3)_2\)) is a salt containing calcium, nitrogen, and oxygen. It is typically used as a fertilizer or in explosives and is not found as a primary component of pearls.
  • Calcium sulphate: Calcium sulphate (\(\text{CaSO}_4\)) is a salt containing calcium, sulphur, and oxygen. It is found in minerals like gypsum and anhydrite and is used in building materials (like plaster of Paris) and other industrial applications. It is not the main constituent of pearls.

Based on the chemical composition of natural and cultured pearls, calcium carbonate is overwhelmingly the main constituent.

Substance Chemical Formula Main Role in Pearl?
Calcium carbonate \(\text{CaCO}_3\) Yes (Main constituent)
Calcium oxide \(\text{CaO}\) No
Calcium nitrate \(\text{Ca(NO}_3)_2\) No
Calcium sulphate \(\text{CaSO}_4\) No

Therefore, the substance that is the main constituent of pearl is calcium carbonate.

Revision Table: Pearl Composition

Component Approximate Percentage Role
Calcium Carbonate (\(\text{CaCO}_3\)) 90-95% Forms the crystalline structure (nacre/aragonite)
Organic Matrix (Conchiolin) 4-8% Acts as the 'glue' binding crystals; provides framework
Water Balance (typically < 4%) Present in the organic matrix

Additional Information: Pearl Formation and Types

The formation of a pearl is a defense mechanism for the mollusk. When an irritant, such as a parasite or a piece of shell, enters the mollusk, the animal secretes layers of nacre around it to protect itself. Over time, these layers build up to form a pearl.

There are different types of pearls:

  • Natural Pearls: Formed spontaneously without human intervention when an irritant accidentally enters the mollusk.
  • Cultured Pearls: Formed when a nucleus or irritant is intentionally inserted into the mollusk by humans. The mollusk then secretes nacre around this inserted material. Most pearls on the market today are cultured pearls.

The quality and value of a pearl are judged based on several factors, including size, shape, color, luster, surface quality, and nacre thickness.

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