The Firmstone Armor was very consuming of magic power.
From Liszt's observations, if Dou Qi releasing a Rock Spike required 1 point of magic power, releasing a Rock Grenade required 5 points of magic power, and releasing a Rock Wall needed 7 points of magic power, then releasing Firmstone Armor would need at least 20 points of magic power.
"This probably has to do with the degree of condensation of the rock, the rock of the Firmstone Armor is almost to the point of crystallizing into gemstones; whereas the rocks of the Rock Wall, Rock Grenade, and Rock Spike are just rock."
A quantitative change leads to a qualitative change.
The more magic power consumed, the higher the quality of the rock can be.