Are spells that do only bludgeoning, piercing, or slashing damage considered magical or nonmagical damage? Most especially when considering them for purposes of resistance/immunity against "bludgeoning, piercing, and slashing from nonmagical weapons that aren't silvered".
For example, Thorn Whip I can see two sides:
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It is being created by a spell, using a melee spell attack to hit, and the spell damage increases with level. Thus it is magical damage.
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The spell description only states piercing damage, and it's the object created by the spell that does the damage rather than the spell directly. Thus it is nonmagical damage.
Similar arguments can be made for other spells (such as Earth Tremor), or portions of other spells (such as the piercing damage of Ice Knife). I'm interested in the rule(s) for this class of spell damage, rather than the specific spells.
[1]: Thorn Whip spell can be found in the Player's Handbook.
[2]: Earth Tremor, and Ice Knife are spells found the Elemental Evil Player's Companion.
Best Answer
For the purposes of the resistance/immunity example in the original question:
The "weapons" portion of the example is actually sufficient to know that it does not apply against the example sources of damage, magical or not. The resistance only applies to specific damage type subcategories of weapon damage. None of the given examples are weapon attacks, so this resistance/immunity to damage from weapons does not apply to the given non-weapon examples in the question regardless of what type of damage is being done.
That said, errata has updated most (all?) instances of the given resistance/immunity example in the question to the following:
(Weapons -> Attacks)
Now it's clearer that the magical status of the attack itself should be considered.
The Sage Advice compendium provides the following checklist for determining if something is considered magical (see also: How do I know if an ability is magical?)
Since the question's examples are all spells or spell attacks, they fall cleanly under the umbrella of being magical. As such, thorn whip and the initial spell attack of ice knife are considered magical attacks, which clearly bypasses the errataed resistance/immunity text. Damage dealt as an effect of spell that isn't related to a spell attack roll (like the damage of earth tremor) is both magical and not from an attack, so the resistance/immunity text also doesn't apply there.
It's also worth noting that there's currently no such thing as an attack that is both a spell attack and a weapon attack, as noted in answers to a question about how to refer to non-spell attacks.