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The Ruby Weave Gem from Fizban's Treasury of Dragons has a feature that allows you to learn a spell from any class list when you finish a long rest:

When you finish a long rest, choose a spell from any class list. The spell you choose must be of a level you can cast. You know the chosen spell and can cast it with your spell slots of the appropriate level until the end of your next long rest.

It seems that the only criteria given for learning a particular spell are:

Obviously, cantrips appear on some class lists, and a cantrip is of a level I can cast, as the rules for cantrips state:

A cantrip's spell level is 0.

However, this sentence makes me wonder if I can actually cast a cantrip learned with this feature:

You [...] can cast it with your spell slots of the appropriate level

Casting a cantrip does not use spell slots. Does this statement require that the chosen spell be of a level that uses spell slots when cast, i.e. 1st or greater?

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You can learn cantrips with the Ruby Weave Gem

Note the following in the item's description (emphasis mine).

You can cast it with your spell slots of the appropriate level.

The key word in the description is can. As in many other 5e descriptions, "can" indicates that you have the choice to use perform an action if the circumstances allow.

In this case, because you can never have spell slots of the appropriate level for a cantrip, you can never cast the cantrip using a spell slot. Nonetheless, this has no effect on your ability to learn the cantrip. As you have stated, a cantrip is a spell of level 0, so all of the other qualifying factors are met.

But can you cast it?

I can find no reference in the rules for cantrips which indicate that you can know a cantrip without being able to cast it (barring specific rules for things like Rage and Antimagic Fields). Therefore we apply the general rule that you are able to cast spells that you know.

Before a spellcaster can use a spell, he or she must have the spell firmly fixed in mind, or must have access to the spell in a magic item.

What about the last phrase?

"Can cast it with your spell slots" only declares the possibility of using your own spell slots to cast it, however it does not make it an explicit requirement. If the intent of the item was that you could only cast the spell with leveled spell slots, then the phrasing would use "must", "can only", or some equivalent wording.

In this case the "possibility" is irrelevant because you already cannot cast cantrips with spell slots. This is similar to how the additional text in Eldritch Smite is irrelevant if the targeted creature is immune to the Prone condition. You can still use the feature, you just can't benefit from the additional rider.

and you can knock the target prone

In the case of Ruby Gem Weave, you can learn the cantrip (and therefore cast it), you just can't cast it with spell slots.

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    \$\begingroup\$ Can you add an explicit statement on whether or not you can cast the cantrip? "you can never cast the cantrip using a spell slot" does not tell me if I can cast the cantrip. A raging barbarian can never cast a cantrip using a spell slot, so that fact alone does not tell me if I can cast the cantrip. \$\endgroup\$ Oct 28, 2021 at 16:13
  • \$\begingroup\$ @ThomasMarkov I came to see your point just as I re-read my answer. Added a small qualifier at the end. \$\endgroup\$
    – Andrendire
    Oct 28, 2021 at 16:20
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    \$\begingroup\$ @ThomasMarkov as I've said, the spell gives you access to the spell which is the sole condition for being able to cast a cantrip. The rest of the text is irrelevant because spell slots do not apply to cantrips. Same way that Eldritch Smite doesn't need to add an additional text to explain what happens when you try to knock Prone a creature which is immune to the Prone condition. \$\endgroup\$
    – Andrendire
    Oct 28, 2021 at 16:31
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    \$\begingroup\$ I get that it is the "sole condition for being able to cast a cantrip", as you put it, but this is under the usual circumstances. My question is if this creates an unusual circumstance, a specific rule, that prevents it as an exception to the general rule you mention, and I don't see that you have made a case that it does not do that. But maybe I'm just misunderstanding you; I'm just giving you my thought process here, so that maybe it will spark some inspiration that can cut through my uncertainty. \$\endgroup\$ Oct 28, 2021 at 16:35
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    \$\begingroup\$ @ThomasMarkov okay added a bit more deliberate wording. Hope this helps. \$\endgroup\$
    – Andrendire
    Oct 28, 2021 at 16:52
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No

The statement says clearly (as you already noted) that you should use a spell slot. The word can says that you can cast the spell or not. For example, if the character chooses the counterspell, he/she may choose not to use it (if the party doesn't face a spellcaster, for example). Additionally, the PHB states cantrips and spells separated when talking about knowing them (for example, the Warlock table contains two columns: cantrips known and spells known).

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