What is a tortoise shell?

I am going to assume you meant either 'what is it made of' or 'how is it formed' so I will answer this way to hopefully answer your question either way...

It is actually a part of the skeletal structure of the turtle and grows with the turtle as it grows, much the same as our ribs (which is what the shell of a turtle is) would grow when we grow. It is mostly bone with some cartilaginous growth. This is why we recommend you do not use paints, glues, or markers to decorate a turtle since it can weaken or even damage the shell, maybe not immediately but over time it could cause great harm to the turtle or tortoise...