When explaining the nature of magic to Eragon, Brom uses the example of a master who, using the word for water, could create a gemstone, because the connection between water and the stone was clear in their mind (an early clue that magic is somewhat untethered from the Ancient Language, incidentally):
“A true master could just say water and create something totally unrelated, like a gemstone. You wouldn’t be able to understand how he had done it, but the master would have seen the connection between water and the gem and would have used that as the focal point for his power.”
On the other hand, Brom also explains that the universe exacts a toll for using magic: the energy of the user.
He leaned forward, eyes gleaming. “Until then, I will say this to discourage any experiments: magic takes just as much energy as if you used your arms and back. That is why you felt tired after destroying the Urgals. And that is why I was angry. It was a dreadful risk on your part. If the magic had used more energy than was in your body, it would have killed you. You should use magic only for tasks that can’t be accomplished the mundane way.”
Magic can be used to accomplish things that would require manipulations that are physically impossible for a human, elf, dwarf, or Urgal to perform, but the energetic cost of those manipulations must still be paid.
Later books show some ways to circumvent this limit slightly. The energy can be drawn from nearby plants or animals, at the potential cost of their lives. Energy can be stored in receptacles over time. And Tenga might be working on some funky solar-powered magic. But the basic limitations remain: you need an energy source.
The energy cost of creating any decently-sized object from energy is, of course, extraordinary. A tiny 2-carat (0.0004 kilogram) diamond has an energy equivalent to 8.6 kilotons of TNT, half the yield of early atomic bombs. That sort of energy can’t come from the biological processes of a human being, or even hundreds of plants or animals.
How, then, can a magician create a gemstone or other object?