If you homebrew a spell gem to contain your choice of spell, spells that use an attack or save must be customized after adding to a character sheet by editing the spell's "To Hit Bonus" and the "DC override" as described in Spell gem.
The spell level determines the gem’s rarity and the stored spell’s saving throw DC and attack bonus, as shown in the Spell Gem table.
However, there is no mention of spellcasting ability modifier as specified in healing spells such as cure wounds, and some other spells like green-flame blade. Edit: If you homebrew a spell gem to contain your choice of spell, this not automatically added to the damage/effect, you would have to customise the spell's "Damage Bonus".
When casting, should the spellcasting ability modifier be ignored, or use the storer's or the caster's modifier, or the larger of the two if the DM is generous?
Additionally, damage cantrips increase the damage when the caster reaches certain character levels (5th, 11th, and 17th). If you homebrew an obsidian spell gem to contain your choice of cantrip, then the damage automatically depends on the caster's character level. I didn't give that any thought until I started thinking about spellcasting ability modifiers.
I think what would be in line with the general philosophy of 5e design is to use the caster's spellcasting ability modifier. This is because the general rules on spellcasting say that that's what you use when you cast a spell. This item has specific rules that override certain aspects of the general spellcasting rules, but not that one, so we fall back to the general rule.
With Spell Scrolls, you should use your spellcasting ability modifier to calculate the effects of spells like Cure Wounds or Heroism. The same should apply then to a Spell gem.
If you homebrew a spell gem to contain your choice of spell, spells that use an attack or save must be customized after adding to a character sheet by editing the spell's "To Hit Bonus" and the "DC override" as described in Spell gem.
However, there is no mention of spellcasting ability modifier as specified in healing spells such as cure wounds, and some other spells like green-flame blade. Edit: If you homebrew a spell gem to contain your choice of spell, this not automatically added to the damage/effect, you would have to customise the spell's "Damage Bonus".
When casting, should the spellcasting ability modifier be ignored, or use the storer's or the caster's modifier, or the larger of the two if the DM is generous?
Additionally, damage cantrips increase the damage when the caster reaches certain character levels (5th, 11th, and 17th). If you homebrew an obsidian spell gem to contain your choice of cantrip, then the damage automatically depends on the caster's character level. I didn't give that any thought until I started thinking about spellcasting ability modifiers.
I think what would be in line with the general philosophy of 5e design is to use the caster's spellcasting ability modifier. This is because the general rules on spellcasting say that that's what you use when you cast a spell. This item has specific rules that override certain aspects of the general spellcasting rules, but not that one, so we fall back to the general rule.
For me, a Spell gem behaves similarly to Spell Scrolls.
With Spell Scrolls, you should use your spellcasting ability modifier to calculate the effects of spells like Cure Wounds or Heroism. The same should apply then to a Spell gem.