What is an example of a mineral that indicates that a metamorphic rock has formed under great depth and extreme heat and pressure?

An example of a mineral that indicates a metamorphic rock has formed under great depth and extreme heat and pressure is garnet. Garnet commonly forms in high-grade metamorphic conditions, often within schist or gneiss. Its presence suggests significant metamorphic processes, as it typically crystallizes under conditions of high temperature and pressure, reflecting the intense environment in which it formed.