An Amethyst provides +18 Defense when inserted in shields or off-hands, +6 to Strength in body armor and headgear, and +80 to Attack Rating in weapons.
An Amethyst requires level 12 to use. When you socket an item with an Amethyst, you increase the level requirement to level 12, unless it was already higher.
Horadric Cube recipes with Amethyst
An Amethyst will drop from TC Normal Gem, which is accessed earliest through Act 4 Good. Learn more about Treasure Classes here.