What cracks it (the manual): an open flame on the stone, a sudden change of temperature, frozen food on a hot stone, knocks and drops, and water on a hot stone. The help centre adds moisture soaked into the stone.
A replacement: Ooni support sends one for a cracked or broken stone. Send your order number or proof of purchase, a photo, the serial number from the rear leg, and your address (help centre).
A broken stone: wear gloves to lift the pieces out.
Owner reports: a few buyers report a stone that cracked on first use. This is not reported as common.