Could it be the case that spren and gemstones are bound together as variables in a cognitive function that is acting as the focus on Roshar? When spren are imprisoned in gemstones, is that what’s really happening, or are they instead being constrained to the same space as part of such a function?
This is what I think is happening here: neither the spren nor the gemstone is the focus, but they are both components in a kind of complex focus.
I like the way you explain this here, and I have an idea that you may not have considered that builds upon the idea of using a function as a focus. could it be that the composition of the gemstone and the type of spren are 2 of the main variables and the cut of the stone represents the physical form of a geometric equation, this wold explain why the interlude from Way of Kings whare 2 people are measuring fire spren is important. the scene demonstrates that spren are affected by math and measurement. keep in mind that another magic system we have seen is geometrically or shape based