How much gold and how much time does fitted mithril or adamantine armor cost in D&D 5e?

I’m looking for the most widely accepted rules for buying mithril and adamantine armor, including chain, splint, half and full plate. How much gold and how much time for a merchant to craft does each piece of armor listed (8 total) cost, baseline? Non official rules are allowed as long as they are widely used. I’ve scoured many sources like the DMG, in addition to the other official 5e sources.

Are weapons and armor made of Mithral or Adamantine considered magical?

In a recent dungeon crawl, I was ambushed by some grey ooze. The ooze managed to land a couple successful hits on me, which would normally cause my armor to degrade. However, I was using a set of mithral chain mail that I had found in a previous session. I know that the statistics for mithral and adamant items are found in the DMG, which leads me to believe they are magical, but their descriptions don’t really imply that they are magical.

Is the mithral armor considered magical for the purpose of resisting effects like degrading from the ooze? I also know that adamant armor and weapons exist in our game. Would they follow the same ruling as mithral?

Does an Adamantine greatclub always deal critical damage like other Adamantine weapons?

In general adamantine weapons always deal critical damage on a hit:

Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

– Xanathar’s Guide to Everything, page 78.

However, in SKT chapter 6 room 14 (page 154) it says:

She wields an adamantine greatclub. This magic weapon has no bonus to attack or damage rolls.

Does this mean it also does not crit? Or does it still crit like adamantine weapons do in general, but simply has no +n to attack or damage roll?

Also: is this a property of this particular weapon in this adventure? Or is it a property that always applies to adamantine greatclubs?

Outside of D&D lore this would make sense because adamantine is a very light material but is still very strong and can be sharpened. This makes is a great material for piercing and slashing weapons (swords, axes, arrows), but a terrible material for bludgeoning damage. However, I don’t know if this applies in D&D lore as well (I haven’t read it anywhere up untill now).

Could Adamantine armors (and Mithral armors) have magical properties (+1 for exemple) in 5e?

As an ex player of D&D 3.5, I’m a little bit confused about Adamantine armors and Mithral armors. In 3.5e, they are not magical, it just classic armor with a different material. So they can have magical properties.

However, in 5e, they are considered magic items in some way, but not in other (see the first related question’s accepted answer). So that why I’m here to ask “Could Adamantine armors (and Mithral armors) have magical properties (+1 for exemple) in 5e?”


related:

Are weapons and armor made of Mithral or Adamantine considered magical?

Can my players coat a +1 Plate Armor with Adamantine?

My Dwarf smith wants to merge both his Plate Armor +1 and his Adamantine Plate Armor. The idea is to melt Adamantine, obtain the raw material, and coat his +1 Plate with it, thus obtaining a +1 Adamantine Plate Armor.

Would a +1 Adamantine Plate Armor be imbalanced?

Players are currently level 12, and I was thinking of charging 1000g for the task, alongside the consumption of one of the armors. The player would not be able to melt the new armor again, so this would be the final version of the item. I’m aware this is homebrew territory (you can only coat weapons with Adamantine), and my main concern is whether this item is overpowered or not. I can adjust its cost, whether it requires attunement, and possibly other effects/penalties.

Do effects that activate on a critical hit still activate when the hit is against an opponent wearing adamantine armor?

I know from this question a critical hit on adamantine will still hit, but I was curious if the critical hit no longer being a critical hit affects features or feats that depend on it.

An example that came up recently in a kind of pvp session is I had adamantine plate armor, and my friend had Great Weapon Master. One of the effects of Great Weapon Master is:

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

Would a critical hit that became a normal hit still activate the bonus action melee weapon attack?

Improved/Superior critical vs Adamantine Armor DnD 5e

Without adamantine armor a critical hit is an auto hit regardless of AC. Though sage advice ruled that a “Improved/Superior critical” attack auto hits when it lands on 18 or 19 BECAUSE it’s a critical hit unlike in PHB where the nat 20 auto hit is an entirely different wording

So my question is would a champion rolling an 18 or 19 on an attack roll auto hit against adamantine armor? I’m assuming it doesn’t since it would take away it’s critical hit status which is the only reason it auto hits.

What would be the price of Adamantine Plate armor?

I’m already aware, as per Are there price lists for magic items, such as the Bag of Holding?, that 5e was designed with the assumption that magic items are not to be bought or sold willy-nilly. This doesn’t happen to be true in my setting.

I’m also aware, as per Do magic items really cost at least as much to create as they do to purchase?, that crafting new magical items is not a worthwhile effort in settings where magic items can be purchased – creating an item is more expensive than outright buying it.

My issue is different: What if the value of a non-magic item is higher than its magic equivalent? In my specific case, I had an issue with the price of Adamantine Plate Armor and the price of regular, unenhanced Plate Armor. DMG page 135 sets up price ranges for categories of items: Uncommon items would be priced in the 101-500 gp range. Adamantine Armor, which can be any sort of Medium or Heavy armor apart from Hide, is one such item. But Plate armor (which is a sort of Heavy armor) costs 1500 gp (according to PHB page 145).

My ruling on the spot was that the 101-500 gp range was for the reinforcement only, and not for the armor itself, and thus the Adamantine Plate armor would be priced at around 2000 gp. Is there another way of coming to an answer that I didn’t consider?

Does a natural 20 on the attack roll still automatically hit if the target is wearing adamantine armor?

My Fighter (Battlemaster) Warlock (Hexblade) build character has just acquired a suit of +1 adamantine plate armour.

From the description of adamantine armor (DMG, p. 150):

This suit of armor is reinforced with adamantine, one of the hardest substances in existence. While you’re wearing it, any critical hit against you becomes a normal hit.

Is a roll of a natural 20 (which is normally a critical hit) still automatically a hit, despite the critical becoming a “normal hit” because of the armour? Or would the attacker need to exceed my AC in order to score the “normal hit” if (for instance) I cast shield or use the Evasive Footwork maneuver to boost AC?

My AC is 20, so with shield cast it becomes 25; for example, would a goblin with a +4 attack modifier score a hit on a roll of 20 against me? A total of 24 would not be sufficient to “hit” under normal rules, but does the “20 is always a hit” mechanism override this despite the critical being cancelled by the adamantine armour?