Black Vitriol

Black Vitriol is a rare mineral only used for one recipe in the game.

It is not technically considered a gem and cannot be placed within a gem bag.

Source
Black Vitriols are found in Dark Iron Deposits, Mithril Deposits, Ooze Covered Mithril Deposits, Ooze Covered Thorium Veins, and Small Thorium Veins.

Black Vitriol as an ingredient

 * Philosopher's Stone