A noteworthy feature of Amethyst Mountain are the multiple horizons of well-preserved petrified forests and concentrations of silicified wood found within the Lamar River Formation that is exposed on it slopes. These concentrations of petrified wood are known as the Yellowstone Petrified Forest. Extensive exposures on the northeast slopes of Amethyst Mountain comprise a classic study area, known as the Fossil Forest, that has been studied in detail by a number of researchers for more than 130 years.