Residuum

Residuum is a green, glass/crystal-like substance created out of refined Residuum Crystals. A rare crystal that is formed from sources of powerful Magic and can be mined. They are quite rare but are found primarily at the sights of the fallen Arcanum cities as large residuum crystals used to hold up the ancient flying cities of the Age of Arcanum.  

Although not inherently Magical, Residuum is particularly receptive to Arcane energies and therefore may be used to create Magic Items that retain more powerful enchantments, possibly for longer periods of time. 

It can also be utilised to replace and act as other expensive materials and components for Spells and other uses. With 1 gram of Residuum is equivalent to 1 GP worth of Spell Components.