The myth that a pearl begins with a grain of sand entering an oyster is a widely held misconception.  While it's a nice idea that a small irritation can lead to the creation of something beautiful, the reality is a bit different.

In nature, pearls are actually formed when a small irritant like a parasite or organic material accidentally gets trapped inside the oyster's shell. The oyster responds by secreting layers of a substance called nacre around the irritant to protect itself. The build up of these layers creates a pearl. 

On a pearl farm, cultured pearls are created by placing a nucleus inside the oyster or mollusk to create that irritant the the mollusk secretes the nacre to cover the nucleus, creating a pearl. The nucleus gives the pearl a "head start".

The myth of the grain of sand may have come from the notion of transformation of something insignificant turning into a precious gem. Pearls symbolize resilience emerging from adversity as an object of beauty.