If you equip the sphere Let's Rock the lavamancer can generate a shield which also absorbs damage by switching between Harden and Inflame mode. If you have already the Tectonic Breaker sphere equiped and place Let's Rock sphere after it in an sphere slot, then the Let's Rock sphere will not work. This also happens the other way around.
steps to reproduce:
The shield should appear after using the Harden / Inflame skill.