Asmodean Crystal

Asmodean Crystals are presumably a type of magical mineral that allow hellborns to access the human world.

History
Asmodean crystals are first referenced in "The Circus" during "Stolas Sings", wherein Stolas procures a book from his personal bookshelf titled "King of Lust - Asmodeus" and skims through pages detailing Asmodean Crystals.

The pages are written in runic alphabet which, when translated, reveal that Asmodean Crystals are mainly given and registered to Succubi by Asmodeus himself, allowing their users to summon a portal to the mortal realm, which may explain how Verosika Mayday and her posse are able to do so in "Spring Broken".

The book also notes that the crystals are often worn in jewelry.

Trivia

 * In "Unhappy Campers", it is revealed that Barbie's Asmodean Crystal is disguised as part of her wristwatch.
 * It is also revealed in the same episode that Asmodean Crystals are not given exclusively to fullbred succubi, but also to hybrids. As the succubi that had the crystal that Blitzo temporary used is also part-imp.