In my Out of the Abyss game the party has acquired a spell gem. Their first use of it with a spell they imbued was to cast Prayer of Healing (casting time: 10 minutes) in the middle of a battle with a Demon Queen. I was fine with this, as I remembered the usage time for a Spell Gem being 1 Action.
However, one of the players (a GM himself) said, “That cannot be right, the casting time shouldn’t change.” So we checked exact wording after the session. (And compared it to a Ring of Spell Storing for good measure.)
While holding the gem, you can cast the spell from it as an action if you know the spell or if the spell is on your class’s spell list. Doing so doesn’t require any components and doesn’t require attunement.
That seems pretty straightforward. It is limited to casters who would have access to the spell (unlike Ring of Spell Storing), but only a single action. Perhaps Prayer of Healing should not have been allowed to be cast into the gem in the first place?
You can imbue the gem with a spell if you’re attuned to it and it’s empty. To do so, you cast the spell while holding the gem. The spell is stored in the gem instead of having any effect. Casting the spell must require either 1 action or 1 minute or longer, and the spell’s level must be no higher than the gem’s maximum.
That seems quite clear that a Prayer of Healing is able to be cast into a Spell Gem. In fact, I believe it means any spell that is not cast as a Reaction or Bonus Action, since I am not aware of any spell with a casting time of more than an Action but less than 1 minute.
This sent the player off to query D&D Beyond for spells they would not normally be able to cast in the middle of a battle, and specifically (because of the thrashing they had just taken) Hallow, Planar Binding, and Glyph of Warding.
That did give me pause when we read it, and made me wonder if RAW were not exactly RAI. But while I might argue that the target would need to be in range for the duration of the initial cast for Planar Binding, I don’t think that’s RAI. And of those three mentioned, I think only Hallow would have had a large impact on the battle they just lost.
So perhaps it is not so destabilizing as I first thought. Do you think RAW are RAI (or did we misinterpret them somehow?)