Wizards require an arcane focus, usually with 1 or more gemstones set into the focus.
To cast a spell a wizard must focus the arcane energies through the gemstone.
Wizards often have several gemstones, set in jewelry or as ornamentation on their person so as to be able to cast a spell.

Cantrips can be cast with no gemstone, but having one makes it easier.

to act as a focus the gemstone must have a value of 50 gp per level of the spell to be cast.
1st level 50
2nd 100
5th 250
7th 350
9th 450 gold.

Casting a spell through a lower quality gemstone will destroy the gemstone.
Casting a spell through a gemstone of higher quality increases the spell save dc by 1 for every 50 Gp step. Max +5

This replaces any other spell component requirements.