Amethyst is a form of silica mineral quartz. Crystals form in gas bubbles, or trees that were engulfed in lava. Sometimes calcite crystals form with the Amethyst. The color is caused by the presence of manganese oxide impurities. Amethyst has been used as a semi-precious gem for thousands of years, and is the February birthstone. It is valued for its purple color and beautiful crystals. This cluster has a cut base to stand upright and is polished.