Mountains of metamorphic rocks can be found in places where the crust is very old, and has been lifted up by certain orogenesis relatively recently geologically speaking.
The metamorphic rocks take a very long time to form. They form mostly of igneous rocks, occasionally from sedimentary ones, that has ended up deeper into the ground and for a long time have been exposed to higher temperatures and pressures. If a certain orogenesis occurs, the area will gradually be lifter up, thus mountains will be formed. The mountains will be formed from the metamorphic rocks that have been there for millions of years, but have been buried underground.
A nice example of this is the Mountain Babuna in the Republic of Norther Macedonia. It is a mountain that has Precambrian rocks, thus it is a very old crust in question. The Alpine orogenesis lifted this area gradually into the air, and from lowlands, created mountains, thus the old metamorphic rocks were lifted up and are now making the core of the mountain. This mountain contains large amounts of one of the highest quality marble in the world, as well as being the only place in Europe that has ruby, unique in the world for being a pink ruby.
