Mendevian Weapon Training material?

[Markov Spiked Chain]

I ended up with two Mendevian Weapon Traninng boons burning a hole in my pocket.

the boon says:

You receive a masterwork version of the weapon or armor, and you gain proficiency in that specific weapon or armor (but not others of its type.)

Can you choose an armor/weapon made of a special material?
Is there any way to "upgrade" to a special material, or buy one up front to get trained in?

Breastplate proficiency would be cool, but I'd really like to find a way to get the ubiquitous Mithral Breastplate. :)

[Nefreet]

You just get what it states. The regular old "add 300gp for weapon/150gp for armor" type of masterwork.