Fibrolite is related to such minerals as sillimanite, kyanite, and muscovite. It is composed of highly metamorphosed shale, which started out as clays with quartz, calcite, feldspar, and other components.
Fibrolite axes are made from a type of fibrolite found only in an area near Truchas Peaks. Not only is the stone itself distinctive, but the form into which it has been ground is also instantly recognizable. Most noticeable is the sharp blade and the spiral groove where the handle is attached.