The Essence of Amethyst is a crafting material in Diablo III, added in Patch 2.3.0.
It is only used with Kanai's Cube to convert 9 gems into Amethysts of same quality.
Squirt the Peddler sells it, among all other Gem Essences, for 500,000 gold each. The item can be stacked up to 5000.