[Vallen Chaos Valiant]

From the author's own blog, here is what she had to say about physical vs magical durability of materials in WCW. Relevant as we just saw Ayaka and Honoka escape their cell.

http://blog.livedoor.jp/wcworx/?p=8

The relationship between magical and physical:
In the WCW world, the more physically strong something is, the easier it is for magic to affect it. Conversely, the more magically resistant something is, the more fragile it is. Even though there are some rare exceptions, they are only of academic interests.

In the case of Diamonds, because it is a hard substance, magic can affect it with ease. You can magically slice and polish a diamond quickly, easily, and precisely. Diamonds are also really easy to enchant, and as such it is a popular ingredient for making Talismans/charms.
On the other hand, to be easily affected by magic means nearly no magic resistance, and can be easily destroyed magically.
This is why there are enchantments that can raise magic resistance.
Because physical resistance and magical resistance are almost mutually exclusive, there are limits to how they can be stacked. An object has the max capacity for 100% resistance. But if you give it 90% physical resistance, you are limited to only enchanting it with up to 10% magic resistance. And if you TRY to go above the limits the object would be destroyed.
Another point is that the resistance of the object can no longer be altered after you give it some other enchantment. So the resistance value of an object is always administered first before any other kind of magic crafting.
For something like a Diamond, with natural 97 physical resistances, you can only give it 3% magic resistance. And careless enchanting could accidentally go over 3% and destroy the gem. Thus most of the time people don’t bother and just enchant it with whatever they wanted. The enchanted diamond thus stays weak to magic for its lifetime.

Concerning the cape the Witches wear, they are tailored extremely well to be very durable, and then has magic resistance buff cast on them. In general the resistance value is 30% physical and 30% magical for the average witch cape.

For the extremely expensive witch garb, the more expensive they are the closer they are to the theoretical maximum of 100% resistance. Like 40% Physical and 40% Magical resistance. A 50/50 witch garb is EXTREMELY hard to produce, and require high quality craftsmanship.
Of course, there exists specialised anti-magic clothing with 10% physical and 60% magical resist. Those capes resemble Cardigans.