To add to Baldguy's comments, there ARE some metal armor items that are medable. When you see these particular metal pieces, they will ALWAYS say "Mage Armor" near the bottom of the description of the piece (the info you get when you point at the item with your cursor). You can find both metal armor and studded leather armor (normally not medable) so marked, and if you do find them, you will meditate in them at the same rate as leather armor or cloth.