A wand is used to direct your energy and as such doesn't have to be made of wood. Some do use wood as well as stones/crystals, a metal rod (sometimes with a stone/crystal attached), a athame or blade, their finger, etc... When a stone or crystal is used it is usually because of that stones mystical attributes such as using a wand adorned with rose quartz for love related spells comes to mind as an example. Of course if you were planning to create a wand using wood it doesn't hurt to look up the attributes of different trees and their magickal uses when deciding what type to use. You would want to take into account if a tree is thought to be better suited for a certain type of magick or if it has more neutral/diverse qualities.