Residuum

Existing in older editions of Dungeons and Dragons, residuum is quite literally magic dust; pure arcana crystallized into a physical form. It is most often traded as a sparkling, silvery powder.
Residuum can be found naturally in some locations such as the Astral Plane, or it can be created by disenchanting (destroying) existing magical items.
Residuum can be traded like any other currency, and is very valuable due to its use in artifice and its high cost-to-weight ratio (residuum is described in its gold cost, and even 10,000 gp worth of it weighs less than a gold coin while taking up just slightly more space).
The gold cost of a magic item specifically describes the amount of residuum needed to craft it, rather than the cost of various other materials as described for mundane items.
Disenchanting a magic item produces 20% of its crafting cost in residuum, and the process of disenchanting may require specific tools (such as a magical furnace) or a special ritual to complete.
If you are constructing a Magic Item from scratch, you must provide both the gold cost of the mundane version of the item and the residuum needed for the magic item's rarity.