you may have hit on something grajekk - i am aware of clay manufacture of products have and some experience w clay- the texture malleability clumping variable sizes of clumps within a field of proximate density -being redder when wet and lighter color when dry all fit what we see- and when such is shipped e.g to a tile manufacturer they just pound out the lumps-so that helps explain the different sized particles /lumps

what seemed odd to me was shipping any kind of aggregate seemed odd/unnecessarily expensive due to the general availability of non-iron nonspecific aggregates

but a good quality clay is less common and prized by manufacturers of expensive tile and probably other products besides crafts/sculptors/artists