Ammoniacum or gum ammoniac is a gum-resin exuded from the several perennial herbs in the genus Ferula of the umbel family (Apiaceae).

It is called the gum ammoniac of Cyrenaica to differentiate it from the commercial one used today, and its source plant has been identified as Ferula marmarica native to Libya and Egypt.

[1] The export of the gum ammoniac of Cyrenaica to Europe declined after Arab and Turk domination of the Middle East,[1] but probably continued at least until the 18th century, as Linnaeus still cited Libya and Egypt as the places of origin of ammoniacum in his Materia medica.

"Lump ammoniacum", the other form of the substance, consists of aggregations of tears, frequently incorporating fragments of the plant itself, as well as other foreign bodies.

Ammoniacum has a faintly fetid, unpleasant odor, which becomes more distinct on heating; externally, it possesses a reddish-yellow appearance, and when the tears or lumps are freshly fractured they exhibit a waxy luster.