(c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Metamorphism is not a one-way street, and it can go backwards, or retrograde, as readily as it goes prograde. This was once a garnet-mica schist, produced by deep heat and pressure upon a shale. But where this schist once had garnets are now lumps of nondescript mineral matter, although probably microscopic study of a thin section would show something there. The garnets were resorbed into the rest of the rock; it looks like the mica has been thoroughly recrystallized as well and is now the silky-looking microcrystalline variety called sericite. The specimen comes from Queen Canyon in the northern White Mountains of California, where intrusions of igneous rock probably did a number on the existing rocks like this mica schist.
A rock's history can be seriously affected by retrograde metamorphism. Rocks that have been subjected to extreme metamorphism can lose all outward signs of that process if they spend too much time at intermediate conditions afterward. For example, that is why eclogite, which is very common deep in the crust, is rare at the surface. Studies of ultra-high-pressure rocks may depend on mere outlines (ghosts) of former mineral crystals, or low-temperature minerals that form pseudomorphs of high-temperature minerals.
Retrograde metamorphism is what makes diamonds so rare. Diamonds are made at great depths, and normal geologic processes that might bring diamonds to the surface are so slow that diamonds retrograde to graphite. Only the rapid eruptions that create kimberlites allow diamond to reach us without degrading.