Amethyst is a common gemstone. Amethyst is only available in the color purple. This gem has a vitreous luster, which means glass as appearance. It also has a hexagonal crystal system. On the Mohs Hardness Scale, Amethyst is on the 7 scale, near defining mineral (10). This mineral has the pleochorism phenomenon, which is when an object (not just gemstones) is rotated, the object changes color.