What was one thing Merlock, a trader said about Eragon's stone?

b. It was herder than any rock he had ever seen

Merlock grimaced. “Unfortunately, neither do I. But I can tell you this much: the white veins are the same material as the blue that surrounds them, only a different color. What that material might be, though, I haven’t a clue. It’s harder than any rock I have seen, harder even than diamond. Whoever shaped it used tools I have never seen—or magic. Also, it’s hollow.”