My Gnoll Paladin just got the Gauntlets of Ogre Strength from Phandelver the other day, and it got me wondering:
A magic item's magical properties begin and end with the effects described in their writeup, plus some general properties that all magical items have; otherwise, they behave like normal objects of their type. So beyond increasing the user's Strength, the Gauntlets of Ogre Power behave like regular gauntlets.
Punching someone while wearing gauntlets is treated like any unarmed attack: all characters are proficient in it, and it does 1 + Strength modifier damage (granted, the Gauntlets help a lot on that Strength bonus part).
Note that there isn't anything that specifically says a gauntleted punch isn't any different from an unarmed attack, but since there isn't anything that says it is, you go with the more general rule.
A DM may want to rule instead that the gauntleted punch is an improvised weapon instead of an unarmed attack, so it'd be 1d4 + Str for the damage (technically, you'd do that if you picked up a gauntlet and slapped someone with it), but even then, the Gauntlets of Ogre Power aren't weapons, so they can't be magical weapons.
The comparison here is like what would happen if you picked up some other non-weapon item and smacked someone with it (except staves, which specifically state that they can be used as a quarterstaff). Whacking someone with a Belt of Dwarvenkind is certainly a fun image, but it isn't a weapon.
The idea of a desperate hero picking up a totally inappropriate magical item because they're up against a monster with immunity is kind of fun, so a DM may well rule the exact opposite of what I said above. It's not like choosing to use the Gauntlets of Ogre Power is going to be an unbalancingly powerful option, since literally any other weapon in their hands would do at least as much if not more damage.
To make a "magical attack", you have to meet one of three conditions:
... a magical attack is an attack delivered by a spell, a magic item, or another magical source
An improvised item is potentially a magic item, but it is not necessarily a magic weapon. Jeremy Crawford has stated:
A magic shield is not a magic weapon, unless its text says otherwise.
That says nothing about being a magic “improvised” weapon or not. It may be the same to him but either way it is unofficial or RAI at best.
The PHB describes an improvised weapon as the following:
Sometimes characters don’t have their weapons and have to attack with whatever is at hand. An improvised weapon includes any object you can wield in one or two hands, such as broken glass, a table leg, a frying pan, a wagon wheel, or a dead goblin.
A set of Sending Stones in the hand would be in line with the "improvised" portion of this. Sending Stones being used to hit someone in the head would be the same as regular rocks. Would this make it magical attacks? Yes, based on the description of magical attacks laid out earlier. Sending Stones are a magic item and can be improvised.
As it concerns overcoming resistance, the text on monster stat blocks state:
Damage Resistances Bludgeoning, piercing, and slashing from nonmagical attacks
This does not limit the attack to just weapon attacks.
In conclusion
Using a magic item as an improvised weapon laid out by the rules would overcome resistances.
Note: Sending Stones are being used as an example and there are many other magic items that would apply.
The cantrip "Magic Stone" would be pretty useless if the cantrip "Light" cast on a stone (or sling bullet) would do almost the same, except the throwing range and spellcasting ability instead of STR or DEX.
But think about a monk, who has ki-powered strikes but needs a later feature to make its unarmed strikes count "magical". Or the magical Wild Shape feature of the druid, which needs a later feature to make the natural beast form attacks count "magical". And the natural attacks of a polymorphed humanoid are also non-magical ones.
If the magic of an item is doing something not directly to cause damage, than the improvised attack with a magical item does not count as "magical" to overcome damage resistance. Like a magical shield +1 does +3 to AC for protection, but the same shield thrown does only 1d4 + STRmod non-magical bludgeoning damage without any +1 from the shield's magic or the ability to overcome damage resistance.
