Amethyst Mountain consists of a geological formation known as the Lamar River Formation. Within the Amethyst Mountain area, it is over thick and consists predominantly of conglomerate and lesser proportions of tuffaceous sandstone and siltstone. Volcanic breccia is absent and only a very few thin airfall volcanic ash beds have been identified in the exposures at Amethyst Mountain. The conglomerates consist of a variety of mudflow deposits (lahars) and braided and meandering stream deposits.