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Could Rust monster or maybe Gray ooze make adamantine weapon rust? Or maybe adamantine doesn't rust at all?

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Adamantine is a type of ferrous metal so it will be affected by Monster features which corrode and rust, including the Rust Monster's Antennae feature

The "Adamantine Weapons" section of Xanathar's Guide to Everything (page 78) states:

Adamantine is an ultrahard metal found in meteorites and extraordinary mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons.

This shows us that Adamantine is a type of metal. The Rust Metal trait of the Rust Monster states:

Any nonmagical weapon made of metal that hits the rust monster corrodes. ..

The Corrode Metal trait of the Gray Ooze states:

Any nonmagical weapon made of metal that hits the ooze corrodes...

Neither the sections on Adamantine nor these abilities make any specific exceptions to this so we would follow the general rules. Both of these features affect nonmagical metal weapons and as Adamantine is a type of metal and is also not inherently magical, the weapons will be affected by both of the Monster's features.


There is also the Rust Monster's Atennae feature which states:

The rust monster corrodes a nonmagical ferrous metal object 

The rules themselves on Adamantine don't tell us its composition; however, user @Doval points out in their answer that the Rust Monster's own statblock states:

Rust monsters roam subterranean passages in search of ferrous metals such as iron, steel, adamantine, and mithral to consume...

This shows us that Adamantine is, in fact, a ferrous metal and so will be affected by the Antennae feature.

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    \$\begingroup\$ Wow, mithral is ferrous? I had always equated it with Tolkien's mithril, which is argentiferous. \$\endgroup\$ – keithcurtis Sep 18 at 13:59
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    \$\begingroup\$ It's probably both, and the rust monster is attracted to and is able to consume the ferrous component of mithril leaving behind silver dust. \$\endgroup\$ – Escoce Sep 19 at 13:19
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    \$\begingroup\$ I always figured they fell under "magical weapons!" That's neat! \$\endgroup\$ – Erin B Sep 19 at 13:24
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    \$\begingroup\$ The idea of an animal that poops silver sounded cool, up until the mithral-destroying part... \$\endgroup\$ – Harper Sep 20 at 19:42
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Adamantine is a ferrous metal, at least for the purposes of a rust monster. From the rust monster's description(MM, 343):

Underground Scavengers. Rust monsters roam subterranean passages in search of ferrous metals such as iron, steel, adamantine, and mithral to consume.

So a Rust Monster's Iron Scent will detect adamantine items of any kind, their Rust Metal trait will corrode nonmagical adamantine weapons and ammo, and their Antennae action can corrode any nonmagical adamantine object. Likewise, a Gray Ooze can corrode nonmagical adamantine weapons.

Whether nonmagical adamantine naturally rusts as easily as iron is unspecified.

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One ability will work, the other won't

Although the ability is called "Rust Metal", the text indicates that the weapon corrodes rather than rusts. The exception being magical weapons, but adamatine weapons are not magical. Xanathar's Guide is the only place they are mentioned and this is what it says about them:

Adamantine is an ultrahard metal found in meteorites and extraordinary mineral veins. In addition to being used to craft adamantine armor, the metal is also used for weapons.

Melee weapons and ammunition made of or coated with adamantine are unusually effective when used to break objects. Whenever an adamantine weapon or piece of ammunition hits an object, the hit is a critical hit.

The adamantine version of a melee weapon or of ten pieces of ammunition costs 500 gp more than the normal version, whether the weapon or ammunition is made of the metal or coated with it.

Nowhere in that text is it stated that adamantine weapons are inherently magical, so the rust monster's power will work on it.

The Antennae ability of the rust monster is a different matter though.

Antennae. The rust monster corrodes a nonmagical ferrous metal object it can see within 5 feet of it. If the object isn’t being worn or carried, the touch destroys a 1-foot cube of it. If the object is being worn or carried by a creature, the creature can make a DC 11 Dexterity saving throw to avoid the rust monster’s touch.

If the object touched is either metal armor or a metal shield being worn or carried, its takes a permanent and cumulative −1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed. If the object touched is a held metal weapon, it rusts as described in the Rust Metal trait.

It specifies a ferrous metal object. Adamantine isn't iron or a compound containing iron. It won't be affected by the Antennae ability as a result.

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    \$\begingroup\$ This is a very good point... except for one problem... we don't actually know that Adamantium isn't some alloy that contains iron. For example, it could be Grade 38 Titanium Alloy. en.wikipedia.org/wiki/Titanium_alloy (i.e. in the rules, it doesn't say what 'ferrous' means nor does it actually say if adamatium is or is not 'ferrous'). Otherwise this is a reasonable approach. +1 \$\endgroup\$ – Reginald Blue Sep 18 at 12:41
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    \$\begingroup\$ As the answer here points out, the Rust Monster's statblock states: "Rust monsters roam subterranean passages in search of ferrous metals such as iron, steel, adamantine, and mithral to consume." \$\endgroup\$ – Medix2 Sep 18 at 13:39
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    \$\begingroup\$ rusting doesn't really differ from corrosion, it is just a specific form of corrosion through oxidation. When iron combines with oxygen, the resulting molecule is a little bit bigger and doesn't exactly fit where the iron ion previously resided so it get's pulled out of it's position. \$\endgroup\$ – Escoce Sep 19 at 13:26
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Adamantine weapons will rust.

The rust monster (Monster Manual p.262) and gray ooze (Monster Manual p.243) have abilities which corrode any nonmagical metal weapon which strikes them.

Rust Metal. Any nonmagical weapon made of metal that hits the rust monster corrodes. ...

Corrode Metal. Any nonmagical weapon made of metal that hits the ooze corrodes.

According to Xanathar's Guide to Everything, p.78, "Adamantine Weapons", adamantine is an ultrahard metal. Therefore, unless they are also magic weapons, they can corrode.

Exceptions

Rust monster antennae: The rust monster's antennae attack is specifically limited to ferrous metals. The 5e sources do not specify whether or not adamantine is a ferrous metal, and the historic lore on adamantine is ambiguous as to whether or not it is ferrous. The rust monster's attack is described as follows:

Antennae. The rust monster corrodes a nonmagical ferrous metal object it can see within 5 feet of it. ...

Adamantine armor: Armor is another matter. In earlier editions of the game, adamantine armor was explicitly nonmagical, such as in the D&D 3e adamantine breastplate:

Adamantine Breastplate

This nonmagical breastplate is made of adamantine, giving its wearer damage reduction of 2/-.

No aura (nonmagical); Price 10,200 gp.

However, adamantine armor's appearance in D&D 5e (Dungeon Master's Guide p.150) simply places it in the magic items section, with no special rule to define as nonmagical. It can quite reasonably be argued that adamantine armor is a magic item, and therefore immune to the rust monster's antennae attack, though I can see some DMs (particularly those who used to play 3rd edition) making a ruling otherwise.

Also, a rust monster cannot damage something like Daern's Instant Fortress, since that's explicitly a magical object.

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