This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Earth is the smallest and loosest of the alchemical substance groups in this taxonomy, holding three members that share a pre-modern classification rather than any real chemical kinship. Sulfur is a native element, the bright yellow, combustible mineral mined around volcanic vents since antiquity and known in earlier ages by its biblical name, brimstone, central enough to alchemical work that it stood alongside mercury as one of the two substances the tria prima theory used to explain metallic properties generally. Its own entry is careful to separate this mined mineral from the alchemical principle also called Sulphur, a philosophical category for the active, combustible nature of things in general that shares the name without being the same thing, a distinction the group's other two members do not need to make.
Terra Sigillata and the Bologna Stone are both, more literally, earths in the sense of mined clays and stones. Terra Sigillata, sealed earth, is a medicinal clay mined at particular sites, most famously on Lemnos, and pressed into tablets stamped with an official seal certifying it genuine before sale. The Bologna Stone is a barite mineral found near Bologna and discovered around 1602 to glow after exposure to sunlight, one of the earliest phosphorescent substances documented in the West. What the group actually names, more than any shared composition, is the old pre-modern habit of filing native elements, clays and glowing stones together under one broad, earthy heading.