Amethysts can found In the Overworld under The Omber Villages. It is more powerful and rarer than the Silver. You can craft the Amethyst Spear or Amethyst Armor. The sword is planned to be crafted as well. When mining, you will get about 1-2 pieces. Though they are pentagon shaped they are Quartz and are also actually Birthstones like Emeralds, Rubies and Sapphires. Like Silver they are Omber Gems.