Geologically, it would make more sense for the clay and sapphire deposits to be separated. The coarser sediment will tend to separate out earlier (further upriver). Of course, that's not going to stop us if we really want to change things, and alsih2o already posted one real-world example where sapphires are indeed embedded in a clay matrix.
I'm just not sure we want to skew the power structure so significantly, and so far I don't really see what it would add for the PCs, apart from some more powerstruggles behind the scenes...
How about this for a compromise: The clay beds are typically well separated from the areas where sapphires are being found. However, some members of the Glazers guild have discovered a rich deposit of sapphires embedded in one of the clay beds, and are trying to keep it quiet, setting up a black market in sapphires. The PCs could be approached by one of the black market sapphire sellers, or they could stumble upon the heavily guarded claybed, or the Glazers could contact the party to find out how the Brotherhood processes the sapphires to bring out their magical qualities...
I'm just not sure we want to skew the power structure so significantly, and so far I don't really see what it would add for the PCs, apart from some more powerstruggles behind the scenes...
How about this for a compromise: The clay beds are typically well separated from the areas where sapphires are being found. However, some members of the Glazers guild have discovered a rich deposit of sapphires embedded in one of the clay beds, and are trying to keep it quiet, setting up a black market in sapphires. The PCs could be approached by one of the black market sapphire sellers, or they could stumble upon the heavily guarded claybed, or the Glazers could contact the party to find out how the Brotherhood processes the sapphires to bring out their magical qualities...