Also known as the Archangel’s Fire and the First Matter, the Prima Materia is behind the murders in the novel. A substance that will give humans god-like powers, the Prima Materia leads apothecaries to become obsessed with it. Possessing it, and particularly taming it, would allow the owner to command a powerful military weapon. In a time of political tensions, this is important to those with alternate viewpoints, such as Oswyn. Even good men like Master Benedict become obsessed with the Prima Materia; it directly kills Hugh and indirectly kills numerous other apothecaries.

Master Benedict’s last note to Christopher leads him on a quest for this recipe. Overall, the eponymous Blackthorn key is the key to the Prima Materia. It is through Christopher’s search for the recipe that he comes to terms with and integrate some of the novel’s most important themes, including the responsibilities of power, the multiple uses of symbols, and the value of things that are typically overlooked.

Mercury has a double meaning in The Blackthorn Key: It is simultaneously a planet and a substance otherwise known as quicksilver. Sands plays on this double meaning by having Master Benedict use Mercury as a symbol twice: once in the puzzle cube and once in the mural opening the crypt to his secret workshop.