Dirt, or fines, can stick to the aggregate even as the material is being sorted for size. Smaller sizes may tend to have more fines as the smaller material all filters down through the screens. Rain, lack of rain, heat or wind, combined with size of stockpiles, timing of when material was screened all can factor into “cleanliness” of the delivered product.
Rocks aren’t manufactured in a warehouse, they are a marvelous product of nature. They are mined in quarries and scooped from large piles of earth.
When ordering material direct from a quarry, expect to receive some fines with your aggregate. Industry standard is about 20% fines on dryscreened material. Our recommendation is to hit the material with some water after installation or wait for a good rain to allow rinsing of the product for the true colors to show.