First some complaining:
I dont like the sound of "significant armor class boost". :-/
Im sorry, but i just dont want to end in Act III. in armor that will give me 28AC, just bcs i have found some "significantly boosted armor" in first part of Act I ... and the game still need some sense of progression after! :-/
I dont also like the sound of "breaking their items" either, both arms and armor ...
Even tho there is said (and yes i have oversighet it at first) "This would apply to non magical weapons only."
Im sorry again, but even tho my original argument about loosing precious loot crumbled to dust, now when i read it properly (

) those items are still loot for me, and potentialy could be used by some of my party members ... also, the option (chance?) alone that our enemy will complely loose its weapon without any chance to recover it ... well, it sounds cool, that much is true ... but also ridiculously overpowered. :-/
Im not quite sure if regular helmets needs to provide any damage reduction honestly ...
The reasoning is the same as with crit protection ... if the strike was not aimed for head, how would you like to reduce anything? :-/
Saving throws should be more than enough in my opinion ... i mean come on, we are talking here about "common" items after all. -_-
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Then some praising:
I like the idea with disarming ...
I would even get so far that every subsequent hit to Admantine armor could have higher and higher chance for that disarm ...
I certainly like a lot the idea that regular helmets should not protect against criticals ... and even more that idea that this should be exclusive for Adamantine gear.
This indeed sounds like ridiculously strong atribute for "common" items. :-/
But i think there is some potential in the idea of destroying enemy armor with Adamantine weapons ...
I would just not make it full destruction ... just reduction of AC for enemies ... maybe even stackable with subsequent hits up to *X*
I like a lot the idea with better chance for Critical Strikes ... THAT sounds awesome!
And i would certainly like option to make more differ weapons.
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And a little nuance to the end:
From story perspective this material will allways be Achilles heel ...
As i allready said multiple times, either we get ridiculously strong equipment right at the start ...
Or we get something that will seem ridiculously weak, concidering it is suppose to be Adamantine ...
Therefore i would like to ask Larian to completely rename all forged gear to something really simmilary sounding, but entirely different ... like Adamantite, or Adaminanite ... dunno, i dont have head for names.

And then add some short journal, where either maker or founder of this forge will confess that he was actualy never able to create REAL Adamantine ... but nobody in the underdark ever known the difference anyway, so he only claimed it is it ... and they keep producing this weaker version, since the armor was pretty strong anyway.

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And a question:
Is there any explanation why none of Sharite gear looks even close to that Adamantine medium armor?
I mean we after all find all Moulds by them ... it would make sense (at least to me) that they would make the weapons look a lot more like the armor they are wearing? :-/