The invisibility of the cracks in the rocks foster the illusion of permanence—a false pride of perfection and durability. If the cracks were obvious, the rocks would appear imperfect, weak and at the mercy of the elements. The poem refers to a stubborn pride, impervious and unfeeling.

The poet is comparing this with the way that a person can be hurt and never show it; but their internal damage is such that at some later date they may suddenly crumble.

The poet chose a seal to catalyze the crack—the most natural and innocuous-seeming creatures, feelings, or things can have an effect when opposed to stubborn elements.

The poet thinks people are rock-like and that people must be adaptable or they, too, will break. Rocks in their motionless state seem safe and people sometimes stagnate under a similar illusion of security.

You have almost certainly known people who seemed to be able to deal with what life threw at them, but then suddenly and unexpectedly ‘went to pieces’. This is what the poem is talking about.

-a wise soul