What is pearl? Name the mollusc which provides most valuable pearl.

Explanation

**Step-by-Step Solution:**

1. **Definition of Pearl:**
- A pearl is a hard, round object produced within the soft tissue of a living mollusk, specifically in response to an irritant, such as a foreign particle. It is composed of layers of nacre, also known as mother-of-pearl.

2. **Formation of Pearl:**
- The process begins when a foreign body, like a dust particle, enters the mollusk's shell. The mollusk reacts by secreting layers of nacre around the irritant to protect itself. Over time, these layers build up, resulting in the formation of a pearl.

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