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Residuum is a green, glass-like substance created out of refined whitestone. Although not inherently magical, it is particularly receptive to arcane energies and therefore may be used to create objects that retain more powerful enchantments, perhaps for longer periods of time. It can also be utilized to replace and act as other expensive materials and components for spells and other uses. Whitestone is the primary source of refined residuum, and exports it in small quantities.
Residuum might have been inspired by the element warpstone from Warhammer Fantasy Battle as they both have a green crystalline appearance and magical properties, though the exact magical properties differ.