Could Rust monster or maybe Gray ooze make adamantine weapon rust? Or maybe adamantine doesn't rust at all?
[RPG] Do things made of adamantine rust
dnd-5emonstersspecial-materialsweapons
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Best Answer
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:
This shows us that Adamantine is a type of metal. The Rust Metal trait of the Rust Monster states:
The Corrode Metal trait of the Gray Ooze states:
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 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:
This shows us that Adamantine is, in fact, a ferrous metal and so will be affected by the Antennae feature.