Some ideas I had related to wands & gems...
A gemstone tip that, when attached to a wand, increases the wand's save DC by +2.
A gemstone tip that, when attached, grants a particular metamagic feat applied to the spell on the wand (could be anything from sonic substitution to enlarge/extend etc. but not maximized, empowered, or quickened).
A gemstone tip that, when attached, allows one to drain 2 charges to cast an empowered or maximized effect from the wand (not sure which; might only allow it to be applied to healing wands).
A gemstone tip that, when attached, allows the user's spellcasting stat to set the save DC (instead of minimum, fixed DC).
A gemstone tip that, when attached, adds xd6 force damage every time the spell is cast (not sure what x should be relative to cost).
A gemstone tip that, when attached, makes any damaging effect into either holy or unholy damage instead of energy, force etc.
An oddly-carved gemstone with two holes drilled through it. When two wands are placed in the holes, one can transfer charges from one wand to the other (losing some charges in the process).
What I don't have a good grasp on is figuring costs on stuff like this.