In my fantasy world, the magic system pretty much makes metal armour and weapons obsolete. The magic system's specifics aren't important, just note that the magic system allows the users to 'penetrate' and 'destroy' man-made materials, AND objects that come from the Earth, such as stone. Only living things such as humans, animals, plants and everything produced naturally by said living things can't be affected by the magic.
What natural armour would be best for warfare/protection? Wood? Giant beetle husks? Scales?
The era this is set in is roughly the classic medieval era of Europe.
Though the people who can do magic aren't common, they provide enough danger that if soldiers do encounter one, they wouldn't want to face them defenceless, as their metal armour and swords/spears/axes are 'ignored'.
- The armour has to be strong enough to protect them from normal soldier's attacks, be it from metal weaponry or otherwise, but also light and inexpensive enough to be breathable and mass-produced.
- The magic system was 'introduced' after humans had already developed metal armours, so that type of armour does exist, but most wouldn't risk the danger that could posses by arming themselves with 'non-natural' materials.
[edit:] Leather armour and [edit: removed chemically treated, contradicts] cloth armour would be viable in defending against said magic.
[edit 2:] To be clearer, the users of the 'magic' aren't 'mages' or 'wizards' old or mostly defenceless without their magic like in cliche fantasy, but are more akin to the 'Mistborn' from the Mistborn series by Brandon Sanderson, or the 'Surgebinders' from the Stormlight Archives, where people such as Kaladin are very adept in hand-to-hand combat. These people who practice 'magic' are also warriors and assassins- using the magic in tandem with their combat skills.
Even if you have leather armour that cannot be affected by their magic, you still have to face the user behind the magic- a potentially extremely skilled spearman or swordsman.