I'm running the Out of the Abyss campaign for my players, and I've hit an edge case for the Sorcerer with Quickened Spell metamagic. The character has a Spell Gem (from Out of the Abyss: you stuff a spell into it, and then spend an action later to have the Gem cast the spell at the Gem's level and save DC).
Can a sorcerer cast two leveled spells in a turn, if they use both Quickened Spell and a Spell Gem? Does this generalize to a sorcerer casting two spells in a turn as long as one of them is from activating an item?
Quickened Spell: When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.
Spell Gem: 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.
(I GM by having a small number of hard/deadly encounters rather than a larger number of smaller encounters – because it makes each encounter have significant stakes, and because Out of the Abyss doesn't restrict resting terribly much in the first half of the campaign. So if the sorcerer can go nova with two leveled spells per round for 2-3 rounds, that does a lot of damage without really restricting his ability to cast spells at the next encounter.)