Here are four short rephrasings:
- Shiny pearls start when an oyster traps a grain of sand and coats it with shiny layers.
- Shiny pearls form as an oyster wraps a grain of sand in many shiny coats.
- Shiny pearls grow when a grain of sand gets covered in shiny layers inside an oyster.
- Shiny pearls are made when an oyster surrounds a grain of sand with shiny layers.