Gneiss

From BrWiki

A structure in Berhagen constructed of gneiss stonework

Gneiss is a type of rock formed by taking another kind of rock and submitting it to heat and pressure under natural geological conditions. This process causes the core minerals of the rock to re-crystalize, forming a superior building material.

In addition to gneiss, other types of rock undergoing this process include marble, slate, and schist.