Magic Items

The vast majority of artifacts or enchanted items are made of metal, stone, or similarly durable materials. While there are some that are made of softer woods, they're often temporary charms. This is because mana stresses any object that contains it. A living creature can adapt to it and heal from any damage, but an inanimate object has to be strong enough to resist taking damage or it will eventually break down.
Many wands are made of oak or harder woods, but only if they're for less powerful spells. Any mage attempting to make a wand for a powerful spell will tend to make it from metal, simply because metal can last almost indefinitely. Normal wands made of a hardwood will last for several dozen castings, but not many more than that.
If the ore or creature that the material for an item came from had a potent magical affinity or grew up in a mana-rich area, they gain a tolerance for mana, allowing the item to be enchanted without breaking down. This is why many magi have only one or two robes they favor, the ones they can enchant are quite limited. The military of Selwyn also favors wyvern hide armor for this reason.