If you have an example I'd be interested in a photo or two.
I've been thinking about the unusual surface layer of "aventurine" and why it would be that thick, right on the surface, that color etc. and how that would occur in the glass pot. That led me to a (completely factually unsupported) guess that the machine operator may have been feeding colorant (metal salts) into the pot by shovel (standard method) and could have dropped some on the rollers. When the next few marbles went through the rollers - they would have picked up the loose dry powder - which was then fused to the glass (but only on the surface). This color of "aventurine" isn't the normal color we see in subsurface glass and is far too dense....hard to imagine any other reason it would be on the surface only.
Anyway...thats my unsupported speculation.