[RPG] Do things made of adamantine rust


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

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:

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.