Kill enough big things at once and it pops; kill a province boss and Carl thinks every crystal in the area goes with it.
Carl builds his whole endgame on a gamble about these. When Katia Grim's Crowd Blast and his twin Boom Jugs killed several of the larger Krakaren simultaneously, the two closest crystals popped soon after — either because the kills landed at once, or because the constant killing finally hit some lifetime load limit. He suspects the bigger the Krakaren Juvenile Cloneling grows, the heavier its death sits on the crystal, and that all of this is by design, a way of keeping things fair. He also suspects that killing a Province Boss would blow every crystal in the area — which is exactly what Prepotente was two hours from proving, expecting a chain reaction through the entire system that would let everyone reach the next floor.
Elle McGib confirms what a spent one looks like: still floating above the burnt-out Ghoul Generator at Station 12, no longer glowing, shrunk to the size of a marble. Carl warns the floor to stay at least two stations clear of any Station 12 or Station 72 — the two station types that carry crystals — because he intends to blow them all at once.
A floating crystal above a generator; spent, it is a dull marble-sized bead.
The floor's demolition charges, and the mechanism Carl means to trigger deliberately.