What is the substance oysters use to make pearls?

Nacre is a composite material made of layers of calcium carbonate crystals (aragonite) and a protein called conchiolin. This gives nacre its iridescent sheen and strength.

Here's how it works:

1. Irritation: When a foreign object, like a grain of sand or parasite, enters an oyster's shell, it irritates the oyster.

2. Nacre secretion: To protect itself, the oyster starts secreting layers of nacre around the irritant.

3. Pearl formation: Over time, these layers of nacre build up, forming a smooth, round pearl.

So, while oysters use nacre to build their shells, they also use it to create the beautiful and valuable pearls we know and admire.