Rocks can be that sharp. Happens quite often. Depends on the type of rock. Every type breaks differently. If you're going to try to critique something for realism, it would help to not be completely flawed in your perception of reality.
Ever heard of stone arrowheads or stone speartips? They're made by breaking stone into a point. By hitting it with another rock and letting pieces fall off as certain types of stone naturally break at a point; not by sharpening it. If it can be done by hitting it with a rock and it breaking off that way, then it can certainly happen in the natural environment when something huge collides with it, such as a boulder. If you've ever been to a mountain range, you've probably seen that while they're often rounded, there are still occasionally sharp edges to cliffs.
Not to mention that these rocks are being summoned out of the ground to build a structure, so the boundaries of reality have already far been exceeded. The rocks could end in the shape of a dragon if he wanted. The answer? Magic.