In this medieval-fantasy (Tolkien-esque, except humans are the only race except for some incredibly rare magical ones) world, a few individuals ( have become mages, sorcerers and monks - which are all in-universe terms for the same thing - by using precious stones to channel the magical abilities inherent in every human. The powers available to the magic user vary depending on the inherent elemental connection of the stone; for example, someone with a ruby ring could manipulate fire, or someone with an opal necklace could control water. (A magic user could collect many gemstones and thus use a plethora of powers.)
Because of the relative rarity of the gemstones necessary to wield magic, the number of powerful or skilled mages is very low. This means that mage-on-mage battles are ultra-rare, the stuff of myths.
A group of dark mages (using onyx to wield shadow magic) have now joined together in an attempt to take over part of the world. Any member of the group wants to wear something which a) looks good/inspires fear in enemies/inspires confidence in allies and b) gives them the most mobility and protection for fighting in.
Most importantly they need to keep their onyx somewhere secure on/in their person/attire, because losing that in a fight means almost certain death because all magical control is lost instantly.
Their clothing needs to be obtainable in a medieval-fantasy setting, though it can be quite expensive because the mages have the ability to teleport and thus steal things from the treasuries of key kingdoms in the world.
Preferably I'd like a differing outfit for male and female mages, but this isn't necessary, I just think it'd be more interesting.
What should I have them wear?