Not only this, but it wasn't even his, according to John Dee and the Empire of Angels:
The large obsidian mirror that is often associated with Dee, an Aztec cult artifact dedicated to Tezcatlipoca brought back from the New World after the conquest of Mexico by Cortés, which has long been on display at the British Museum,54 may not have been used at all, nor has it been conclusively shown that the mirror even belonged to Dee. If the black mirror was Dee’s, it may have come into his possession at Louvain or through a Spanish courtier.55 Of the other objects in the British Museum attributed to Dee’s use, it is likely that only the three wax seals actually belonged to him; the crystal ball and gold engraving of the “Vision of the Four Castles” are probably later acquisitions by collectors from other magicians or antiquarians who reproduced them from Casaubon’s printing of A True & Faithful Relation.56
The large obsidian mirror that is often associated with Dee, an Aztec cult artifact dedicated to Tezcatlipoca brought back from the New World after the conquest of Mexico by Cortés, which has long been on display at the British Museum,54 may not have been used at all, nor has it been conclusively shown that the mirror even belonged to Dee. If the black mirror was Dee’s, it may have come into his possession at Louvain or through a Spanish courtier.55 Of the other objects in the British Museum attributed to Dee’s use, it is likely that only the three wax seals actually belonged to him; the crystal ball and gold engraving of the “Vision of the Four Castles” are probably later acquisitions by collectors from other magicians or antiquarians who reproduced them from Casaubon’s printing of A True & Faithful Relation.56