Rocks in these areas will often be subjected to enough stress to form new rocks. Metamorphic rock is not always radically different from the rock that formed it. The same is true for most igneous rocks. Sandstone transforms into quartzite under pressure - but quartzite is just very hard sandstone. Limestone can be metamorphosed into marble, which is recrystallized limestone.
Marble is basically the same type of rock as crystalline limestone.
The rock that is most radically changed by metamorphism is shale. Depending on the amount of heat and pressure present, shale can be transformed into slate, or phyllite, or schist, or gneiss. The last three result from chemical changes which transform clays into micas or quartz or feldspars. Slate - very hard shale. Slate is somewhat brittle, but doesn't break as easily into sheets as shale, and can't be scratched with a fingernail.