Wand Caps are one of the two main components of making wands (including staves and scepters), the other being the wand core.
All wand caps begin with 5 nuggets of some metal, crafted in the "helmet" arrangement (iron shown). However, advanced caps require progressively more complex magic to complete them, and the more advanced the caps, the more expensive it is to attach them to a wand.
The choice of wand cap does not affect the storage capacity of a wand, but can provide a bonus or penalty to all vis costs when using the wand. The bonuses or penalties from caps combine with any other adjustments provided by equipment or other factors.
Some cap types depend on metals from other mods. For example, the Copper and Silver caps will only be available (or researchable) if nuggets of those metals are present in the game.