The baking stone cracked

Makes the pizza unusable

Almost always thermal shock or absorbed moisture rather than heat alone — a stone tolerates very high temperatures well and tolerates rapid change badly.

Safety: A cracked stone can break apart while loaded and while hot. Do not attempt to keep using one, and take care retrieving the pieces once the oven is completely cool.

Ask yourself

Was the stone heated from cold with the oven, or placed into an already-hot oven or grill?
Did anything cold — frozen dough, a splash of water, a cold worktop — contact it while it was hot?
Had it been washed, or stored somewhere it could absorb moisture?
Was it dropped or knocked at any point?

Likely causes

#1
Thermal shock from a rapid temperature change across part of the stone
Strongly supported
Stone expands as it heats and contracts as it cools. When one region changes temperature much faster than its neighbour, the resulting stress splits it — this is by a wide margin the commonest cause.
#2
Absorbed moisture turning to steam inside the stone on heating
Strongly supported
Stone is porous. Water absorbed from washing, or from being stored outdoors, expands violently as it vaporises and can split the stone from the inside.
#3
Cold or frozen food placed onto a fully heated stone
Generally accepted
A frozen pizza or fridge-cold dough ball chills a localised area of a very hot stone almost instantly, producing exactly the localised stress thermal shock needs.
#4
Mechanical damage from a knock, a drop, or uneven support
Generally accepted
Cordierite and ceramic are brittle. An existing hairline fracture from handling will open up under the next heating cycle.

Equipment-specific causes

The same symptom has different answers on different equipment. Each branch below states how to tell whether it is yours.

Equipment

Cooking on a pizza stoneStrongly supported
How you would know: The stone cracked during or shortly after a heating or cooling cycle, or immediately after something cold touched it.
Thermal shock. A stone expands as it heats and contracts as it cools; a sharp temperature change across part of it makes one region expand at a different rate from its neighbour, and the stone splits along the stress.
Do now: Retire the stone. A cracked stone can fail completely under load, and continuing to bake on the pieces risks both the pizza and the person retrieving it.
Change next time: Heat the stone from cold with the oven, never launch frozen or fridge-cold dough onto it, never splash it with water, and leave it in the oven to cool with the door ajar rather than moving it to a cold worktop.
Pizza on a BBQGenerally accepted
How you would know: The stone cracked shortly after being placed onto an already-lit BBQ.
The stone was placed onto a hot grill rather than heated with it. Direct flame beneath one part of a cold stone creates exactly the localised temperature difference thermal shock needs.
Do now: Retire the stone rather than continuing on a cracked one.
Change next time: Put the stone on the grates before lighting the BBQ so the two heat together, and store it indoors — a stone that has absorbed rain will turn that water to steam on heating and split.
Pizza in a kamadoGenerally accepted
How you would know: The stone cracked or scorched on the underside, sitting directly on or very close to the heat deflector.
No air gap between the deflector and the stone. With the two in contact, heat conducts straight into the stone's underside while its top face stays cooler, which is a direct route to thermal stress.
Do now: Retire the stone and reconfigure before the next session.
Change next time: Leave an air gap between deflector and stone — the cooking grid between them, or spacers — and bring the kamado up to temperature slowly rather than rushing it.

Rescue it now

Retire the stone rather than continuing to bake on the pieces — a cracked stone can fail completely under load, which is both a ruined pizza and a genuine burn risk while retrieving it

Prevent it next time

Heat the stone from cold with the oven or grill, never wash it with water or soap, store it indoors and dry, never launch frozen or fridge-cold dough onto it, and leave it in the oven to cool rather than moving it onto a cold worktop

Domestic fan (convection) oven

Go deeper

Guides that treat this as an outcome to improve, an environment to work with, or a flavour to understand, rather than only a problem to fix.

How to get better pizza on a BBQ Cooking on a pizza stone Pizza on a BBQ Pizza on a gas BBQ Pizza on a charcoal BBQ Pizza in a kamado