Problem with the quest " the bronze beneath the lake"

[Rymzz]

I killed the giant slayers but apparently i didnt find the stone needed to open the ruins,even if i took all their items, i still cannot go through it. I have enghwitan relief gem in inventory, wonder if this is the stone recquired. May someone help me please ? ( and sorry for my english )

[sorophx]

you have to place the gem into the socket at night, only then will it open