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

Question

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?

Answer 1

There are magic and non-magic items made from adamantine

While the DMG lists Adamantine Armor as a magic item, some items made from adamantine are not magic.

Gargoyles resist damage from “nonmagical weapons that aren’t adamantine” (MM, p 140). It follows there would be nonmagical weapons that are adamantine.

So, adamantine weapons would strike gargoyles similarly to how magic weapons would, without being magic weapons, or even magic items. Mechanically, they are similar to silvered weapons (the phrasing in monsters’ resistances blocks are the same) but they defeat a different set of monsters’ damage resistances.

Xanathar’s Guide does not call Adamantium “Magical”

Xanathar’s Guide to Everything (p. 78) has a section on adamantium weapons which describes adamatine as “an ultrahard metal found in meteorites and extraordinary mineral veins” but does not mention it being magical.

Xanathar’s lists the properties of weapons “made from or coated by” adamantine. These are distinct from the proprties of magic items. For example, adamantine weapons don’t get magic item resiliency.

Official Example (Spoiler Alert)

There is an example of such a weapon in official Wizard’s 5e material.

In The Lost Mine of Phandelver, the Spider Staff is a “black, adamantine staff” that “can be wielded as a quarterstaff.”

Note, the staff does not strike as a “magic quarterstaff” like the Staff of Power does. It is a magic item but not a magic weapon. As far as whomping things goes, it is a “nonmagical weapon made of adamantine.”

For mithral items, the rules say very little

I’m not aware of any mention of mithral in the core rules outside of the magic armor. A DM might follow the example of adamantine — that it’s a rare metal that is often found in magic items. Or they might rule it is inherently magical.

Magic Items are defined as such. Custom items are defined by your DM.

If an item is listed as a magic item then in general they would have the qualities common to magic items of their type. A custom item invented by your DM might have any properties.

The particular qualities of any particular item in your game are of course up your DM. Your character might (and probably should) learn whether an item is magical or not, and its properties, resistances, etc., by means such as an Identify spell.

Answer 2

Mithral and Adamantine armor appear under the "Magic Items" header of the DMG, and are listed in the magic item tables (Adamantine armor is actually the very first entry on pg. 150). This means that they are magic items, with all the attendant properties.

I don't think there's any listing for adamantine weapons anywhere in the handbooks. Given that the armors made from those materials are magic, it's not a stretch to say that the corresponding weapons are also magic.

Lore-wise, this can be justified by saying that adamantine and mithral are inherently magical materials.