A distinctive feature of the staurotheke is the use of jet – a semi-precious stone, associated in the Middle Ages with magical and healing properties. The paper proposes a new interpretation of the function and character of the staurotheke, based on the possible meanings conveyed by the use of jet. The strong association with the feminine, both of the stone used and of the relics of the Holy Cross Tree, makes it possible to link the reliquary to a female figure from the ruling family. The peculiar electrostatic properties of jet, as well as the modest design and small size of the staurotheke, and it not having been used for propaganda, point to an object of private devotion rather than a diplomatic gift. The summary of the history of jet’s symbolism aims to demonstrate how the tradition of magic and alchemy, which has been passed down since antiquity, can play a major role in contemporary research into the interpretation of the meanings conveyed by the use of precious stones.