You can imply from the title and description that Celestial Armor is armor made by (or at least on the home plane of) Celestials. My take, therefore, would be that the special properties are partly due to material (celestial metal - could be mithral, could be steel, could be gold and silver, etc.) and partly due to environment. So, the individual DM can say that the armor, as is, is made up of anything he wants (just be consistent). A similar argument can be made for Fiendish Armor.
What this also means is that you cannot upgrade a non-celestial armor to have celestial qualities. That would be like saying you want to upgrade your Rhino Hide armor to Dragon Scale armor. That's not an upgrade, that's a replacement.
Other types of Celestial Armor (Celestial Plate, for ex.) would be up to the individual DM, but remember to justify it. I would be surprised to see any kind of winged celestial wearing heavy armor, but the really powerful ones may (such as in various works of art depicting the angels Michael and Samael).