Semi Precious Amethyst StoneHardness : 7.0. Appearance :Violet, lilac or mauve quartz is called amethyst. Amethyst is sometimes heat treated to form citrine. Amethyst crystals can form together with citrine crystals to form bi-colored quartz called ametrine. Amethyst is the birthstone of the month of February. Enhancements :Occasionally heated to lighten color and/or to remove "smoky" components.