Viridian crystal

Viridian crystals, colloquially known as fool's emeralds, was a crystal found in the Underdark and underground locations in the Western Heartlands.

Description
Viridian crystals were known for being hard to work with because of their brittleness. Its crystalline form was sharp-edged.

Powers
Even though sometimes used for decoration and crafting, fool's emeralds had some alchemical properties. These crystals could be transformed via alchemy into salts of viridian crystal that had arcane symbols appear and dissolve in its grains. This substance was mixed with a special vitriol to create a diluted variant of the oil of sharpness.

Appearances

 * Video Games
 * Baldur's Gate III