Stone is an extremely durable substance; it can endure thousands of years through wind, rain, changing seasons, and weather, remaining steadfast and unmoving.
Yet, while hard, it is also fragile. Subjected to enough force, it shatters completely.
The two Death Eaters in He-Who-Must-Not-Be-Named's hands had stiffened completely after being hit with the Petrificus Totalus. Because they were already on the verge of death before being struck, He-Who-Must-Not-Be-Named had held nothing back.
Chapter 321