There have been a few of these lately about problems memorizing specific spells so I figured I’d lay out the rules.
Avatar spells can not be memorized.
Currently there are only mage stones in the game so only mage spells can be memorized. A mage spell is a spell where the mage level to learn is <= all other classes level to learn.
So from the bug reports:
Lloyds Beacon = Cleric Spell
Improved Haste = Avatar Spell
You can memorize spells like “spirit watcher” where multiple classes receive the spell at the same level as mage.