Residuum

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.

Trivia
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.