My GM granted me with a suit of adamantine armor, but I am trying to understand the effect. I searched around for clarification, but I did not find much on it or failed to locate

"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 it just that simple? No more capacity to critical against the wearer? Or is it no doubling of damage, but riders still persist? Does it extend to spells with an attack role as well and their rider effects on crit?

its that simple - no crits when you're hit by an attack - which is the only thing that crits as far as i know.

Yep, that's the only effect. It otherwise functions as a normal suit of armor (of whatever kind of armor it is) but all critical hits (from any source) turn into normal hits instead.

Cool, thank you both. I can't tell if it just feels powerful considering the level of magic item in our low magic campaign or if it will actually play that powerful in game. As a player, I am always leery of tipping power towards the player in homebrew. Having an item which the GM homebrews, but then spends the rest of the adventures regretting makes me ensure that they know what they are handing out. I will just get in their ear to ensure that is the intent of the item and make sure it is cool and does not thwart anything they are planning.

It’s not technically a magic item, just a special metal like titanium.

Quote from theHumanChris >>

My GM granted me with a suit of adamantine armor, but I am trying to understand the effect. I searched around for clarification, but I did not find much on it or failed to locate

"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 it just that simple? No more capacity to critical against the wearer? Or is it no doubling of damage, but riders still persist? Does it extend to spells with an attack role as well and their rider effects on crit?

 It's as simple as it sounds. A critical hit becomes a normal hit. So anything extra that would happen on a critical hit doesn't happen. No doubling of the dice damage, if they have Great Weapon Master they wouldn't get the bonus attack, etc. There are some magic items that trigger effects "when you roll a 20", not "when you make a critical hit", so I believe those effects would still happen.

Quote from IamSposta >>

It’s not technically a magic item, just a special metal like titanium.

I thought that the material Adamantine was just inherently magical, making items forged with it magical as well. If your ruling is correct,  does that mean it would work in a Antimagic Field? 

Quote from JoeltheWalrus >>

Quote from IamSposta >>

It’s not technically a magic item, just a special metal like titanium.

I thought that the material Adamantine was just inherently magical, making items forged with it magical as well. If your ruling is correct,  does that mean it would work in a Antimagic Field? 

 Well, it is technically listed as a “magic item” so I guess I was wrong.

Quote from JoeltheWalrus >>

Quote from IamSposta >>

It’s not technically a magic item, just a special metal like titanium.

I thought that the material Adamantine was just inherently magical, making items forged with it magical as well. If your ruling is correct,  does that mean it would work in a Antimagic Field? 

Adamantine is not an inherently magical metal, nor is a weapon or armor made of adamantine inherently magical. There is a magic item called adamantine armor which is an example of a specific rule modifying a general rule. After all, rust monsters roam subterranean passages in search of ferrous metals such as iron, steel, adamantine, and mithral to consume and we can plainly see they can't rust magical weapons and armor.

Quote from TexasDevin >>

Quote from JoeltheWalrus >>

Quote from IamSposta >>

It’s not technically a magic item, just a special metal like titanium.

I thought that the material Adamantine was just inherently magical, making items forged with it magical as well. If your ruling is correct,  does that mean it would work in a Antimagic Field? 

Adamantine is not an inherently magical metal, nor is a weapon or armor made of adamantine inherently magical. There is a magic item called adamantine armor which is an example of a specific rule modifying a general rule. After all, rust monsters roam subterranean passages in search of ferrous metals such as iron, steel, adamantine, and mithral to consume and we can plainly see they can't rust magical weapons and armor.

 Huh. So can you make armor out of Adamantine that is non-magical? 

I have always treated Adamantine and Mithral as non magical metals. 🤷‍♂️ It never broke anything for me.

There would be no reason to make a suit of armor from adamantine since it would just cost more and bestow no additional functionality to the non-magical armor. With an adamantine weapon, on the other hand, it has special properties when used against objects and against certain monsters (for instance, iron golems are immune to Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't Adamantine--further evidence that adamantine is not inherently magical). You can signify that your weapon is made of adamantine by clicking the weapon on your character sheet, selecting customize, and ticking the box for adamantine.

Quote from TexasDevin >>

There would be no reason to make a suit of armor from adamantine since it would just cost more and bestow no additional functionality to the non-magical armor.

 What? I think you might have missed something:

Adamantine Armor

Adamantine Armor is not armor made of Adamantine, it is magic armor that is "reinforced with adamantine." Texas is correct. 

In prior editions, Adamantine, Mithral, etc. were special materials which imparted specific benefits to anything made out of them, even when that armor/weapon was not enchanted as magical. 5E doesn't have that system, so the only use for non-magical adamantine objects is using them to beat on creatures with DR/adamantine

Fair enough. I still use 2e ideas for that stuff because I still use a 2e setting. All of those are still mundane metals with special properties in my setting so I guess I never noticed before.

IamSposta, I feel the same way. I still think of both as extremely rare metals which exhibit magic-like properties, but are not inherently magic. My old brain is filled with ancient lore from older editions and the effects from those still alter my perceptions of the current. So, Adamantine and Mithral are considered magic armor without a riding enchantment that can be undone by an anti-magic field. That in and of itself keeps it immune to rust monsters as a small perk I guess.

As for how powerful the item is, most of the time it's marginally inferior to +1 armor, but it's also lower rarity. In normal use

• +1 armor will prevent one hit per 20 attacks. If you're fighting orcs, that's 1d12+3 damage prevented.
• Adamantine armor will convert one crit to a normal hit per 20 attacks. If you're fighting orcs, that's 1d12 damage prevented.

Exact ratios depend on the opponent, but on average it's about 2/3 of a hit. It becomes higher value if enemies have advantage or abilities that are triggered by critical hits, lower if they have disadvantage.

I've had interesting conversations with my players on the properties of adamantium, mithril, and other "semi-magic" metals.  One of my player's PC is an artificer with a background/interest in metallurgy and the setting is steampunk industrial-ish, so there's a lot of natural opportunity to work with these metals (I even imported orichalcum from 3.5e any modified its properties for 5e rules)

It reads as strange to me that a metal's natural property causes it to be a magical item in 5e, so we houseruled that adamantine armor and mithril armor were non-magical (Orichalcum remained magical, because its properties affect magic directly, and go beyond being "super hard" or "super light and flexible")

For the record (and I'm aware this was homebrew) this is what I ruled for each

• Mithril armor - properties given in the magic item, but count as non-magical
• Adamantine armor - properties given in the magic item, but count as non-magical
• Orichalcum armor - reduced incoming spells by one damage die, counted as magical
• Mithril weapons - lose the "heavy" property (but still can't be wielded fully by small creatures), or gain the "light" property, depending on the weapon, count as non-magical
• Adamantine weapons - properties given in xanathars, count as non-magical
• Orichalcum weapons - can be used as a spell-casting focus, add one damage die to spells with an attack roll cast using the weapon as a focus, counts as magical.

what source book are you getting orichalcum from? google says its in exandria, but when i look through taldorei campaign setting, i see it only mentioned once but no real description.

I also still use “Red Steel” from the old Mystara setting, but had to houserule my own rules for it:

Red steel is one of the rarest and most valuable metals in the world. It is naturally imbued with magic as it is the material leftover from the depletion of Cinnabryl, a substance found only in the Savage Coast. Any equipment made from red steel has the following properties: 

• Equipment made of red steel weigh half the normal weight for items of their type.
• Any weapon made of red steel counts as magical for the purpose of overcoming damage resistance.
• Equipment made of red steel cost half the normal investment of time and gold to enchant. 
• If the full cost and time are invested to enchant a weapon, shield, or armor, the item automatically gains either an additional uncommon feature, or an additional +1 bonus, determined by the DM at the time of creation.
• If an item made from red steel is imbued with an Artificer Infusion, the infusion becomes permanent and does not count against the characters number of infused items.