Simplify the following text for a Grade 8 native English-speaking person.
Original: The harder forms , such as anthracite coal , can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure .
Simplified: Coal is a metamorphic rock formed from peat by the pressure of rocks laid down later on top . The harder forms of coal , such as anthracite , have been changed by higher temperature as well as pressure .