I like the idea of the veins being pure in terms of their 'useful' product, but the stone being a result of crushing and sorting. A bit like how it's done in Angel's Refining, just with the crushers abstracted as a component of the vanilla miners. I tried editing the minimum_value myself in settings.lua and it seems to generate ore patches correctly with stone as the only impurity when the stone byproduct setting is enabled, so I'd like to suggest you expand the range too so anyone else looking for this can easily configure it this way.