Book about people turning to stone from power use

Question

I can't remember much about this book hence why I wanted to re-read it but its power cost is unique enough to set it apart.

People had this magic, very powerful but it had a hefty cost, any spell no matter how small turned a part of the casters body to stone. They could move regularly until they used it too much to the point they were 100% stone then they became a statue.

Another big thing I remember is the main character (a male kid) saved some girl from a basement and turns out she was some sort of entity. The girl in return gave the kid every spell created. So he knew how to do a bunch of spells but the cost was the same.

Answer 1

So, as @JohnRennie mentioned in comment, this sounds a lot like the Broken Earth trilogy by N.K. Jemisin.

Specifically, I think you were reading the second book in the trilogy, The Obelisk Gate, which has pieces and fragments of what you describe, just not quite matching all the way.

• Rare people called Orogenes have a kind of Earth magic that can unleash or quell earthquakes at will, moving the energy around. At a certain level of strength, that magic starts resonating with the Earth and starts turning the user's body to stone. They are severely persecuted and enslaved, ostensibly to keep the world from ending.
• One of the two main characters, Essun, a mother and orogene looking for her daughter, takes refuge in an underground town of sorts. It's a lot like a geode, and has a false town on top of it, with secret entrances from some basements.
• Alabaster, a sometime ally orogene and with a lot of history with Essun, has basically started the end of the world, in an attempt to fix it. This has kickstarted his turning to stone, and he spends most of the book in an infirmary. Essun over the course of the book also hits this point.
• Alabaster has an ally, a Stone Eater, who has a female form and has provided him with a lot of information on the history of his art and skills.

Not really matching:

• The book cover does not have a hammer, but some ornate stonework, at least in the versions I find.
• Alabaster is never really a main character, although he does figure prominently in Essun's story and history.