Could anyone tell me what special item is required to move the giant boulders blocking off access to a treasure chest?

Deep in the Spire and you'll have to swim underwater to reach the area is an idol (I think it might have been of Earth) whatever. When you reach it and go through the converstational bits, the idol will lower those boulders blocking your path.

Wyv

Actually, I think the Guardian gives you a ring which you can 'use' on the boulders. This causes them to sink most of the way into the ground so you can get over them to the chest (this is another 'tight spot' where I had to jump over the rocks using my Pixie as lead in order to get by them too).

thank you both for your assistance.