Schists are a type of rock that is formed from other rocks under heat and pressure.
Schists are strong and hard because they are formed under heat and pressure.
Examples:
The mountain is made of schists.
The miners found some shiny schists.
She collects different kinds of schists.
The schists are very old and very hard.
The schists are as strong as diamonds.
The schists are very heavy and very rare.