Aethiops Mineral is identified as mercuric sulfide, HgS, chemically identical in composition to red cinnabar and vermilion, but crystallizing in a different, metastable structural form usually called metacinnabar in modern mineralogical terminology, a genuine and confirmed case of two chemically identical compounds differing in physical form and appearance because of how their crystal structures form under different conditions, direct low temperature combination for the black form, versus the higher temperature or natural geological conditions that favor the stable red form. This is directly comparable to, and worth reading alongside, this atlas's entry on cinnabar, which notes the same structural duality from the opposite direction; aethiops mineral is best understood not as a separate compound from cinnabar but as the same chemical formula realized in a different, less stable crystal arrangement, a distinction alchemical chymists could observe reliably through color and texture even without the crystallography to name what they were seeing.
Aethiops mineral's black color also gave it a memorable place in the broader symbolic vocabulary of alchemy, where blackening, nigredo, marked an early and necessary stage of the Great Work, an association this entry notes as symbolic context rather than as any claim about the compound's actual chemical role in that scheme.