There was more to the concept Ambros used when he molded the needles. Because of his in-depth understanding of Earth magic, he could manipulate the rubble's composition and create an entirely different material. That was how he made those needles as sharp as a blade and solid as steel.
For the record, the arena was made from a durable material. That was how it could withstand most explosions. Because of its durability, it would be close to impossible for someone to create a sharp weapon from it. They would need to break the material, heat it, and forge it properly to have such a result.
The concept was similar to how Alchemists concocted potions. Depending on how they ground and combined ingredients, they could produce either medicine or poison.
