This opens up the possibility that a previous user could have used this power as a sort of “cryo sleep” for an important figure or figures in the past. Maybe a collection of giant warriors that they could unpetrify if and when they needed them? This is why the World Government really fed Hancock the Mero Mero, because they use slaves to do their dirty work for them.

This explains Blackbeard’s whole journey. He originally went after Hancock to get this fruit, even ready to sacrifice some of his crew for it. Then here he saw the Pacifista of Boa and realised she had the Mero Mero ability too. Once Rayleigh turned up, Blackbeard figured it’s not worth the effort to fight them and backed off, kidnapping Coby in the process to find out more about the Seraphim.

Either Koby spilled or they used Pudding, but they found out about the authority chips. Blackbeard then hears about the commotion over in Egghead, so he sends in Devon and Augur who have powers perfect for infiltration and escape to either steal an authority chip or have Devon copy Vegapunk. Luck would have it they found a Gorosei just on the outskirts of the Island and their mission was complete. They can now use Devon to control the Seraphim, thus effectively giving them the power of the Mero Mero in their crew.

Blackbeard can now use this power to unpetrify someone. It could even be Rocks, hence the name. Hell, if it was those giant statues in Mary Geoise it could explain his plan for Hachisou to be a World Government nation. Access to Mary Geoise – similar to what he did in Impel Down.