The sentence you wrote is almost correct! Here's a slightly better version:
Original: When a grain of sand gets inside an oyster’s shell, it is covered with many shiny layers and turns into a big pearl.
Corrected: When a grain of sand gets inside an oyster’s shell, it is covered with many shiny layers and turns into a big pearl.
Actually, your sentence is already good! If you want, you can also say:
When a grain of sand gets inside an oyster’s shell, the oyster covers it with many shiny layers, and it turns into a big pearl.
Both sentences explain the pearl-making process clearly and are grammatically correct!