This variant goes one step beyond a Magic Point or Spell Point system to link those points to physical things: mithril, and it's various manifestations. In this magic variant spellcasters have to gather supplies with which to power their spells, whether that means purchasing them, slowly distilling their fuel from the environment, theft, or foraging. 

This variant results in less powerful spellcasters with limited resources, and slows the pace of play. It's most suitable for a game that's using slower rests (e.g., a 'short rest' takes place overnight, a 'long rest' requires a week or a campsite).