Earth elemental core
You can replace material component of Conjure Elemental spell to create an Earth Elemental
and the created elemental retains its memories as long as you use this exact core as material component each time
you conjure earth elemental.
It is usefull if you need this elemental knew something before it was destroyed or dispelled and
it might also not attack you or your allies if you lose Concentration during Conjure Elemental spell
(if you befriended it and have friendly relationship).