The creation of a pearl is remarkably simple to understand, but fairly complex and lengthy process to create. A pearl is formed when an irritant, such as a piece of sand, becomes lodged in the shell of an oyster. Sensing the object, the oyster deposits layers of a semi-translucent substance called "nacre" around the intrusive piece, where it builds up over time. It commonly will take years to create a pearl of decent size and perfectly round shapes are extremely rare. In fact pearly jewellery made with perfectly round pearls is quite expensive.