It makes sense to enhance appearances but a gallon size baggie of the superfine glass weighs about nothing. Glass that would add appreciable weight is called sand. I'm four years from smoking street/dispensary weed. I know I've smoked some contaminants in my day but what are you going to do? I'm going to twist up one out of some shitty ol' Colorado homegrown that I nurtured from day one.I've done some fiberglass work too, and while it would be obvious if somebody chopper-gunned your buds, there are lots of smaller fibers and fillers used in that industry that could give just an effect as stated. Microspheres and silica are two that come to mind.
Surely the urban scare legend gets a lot of mileage beyond reality, but still makes me realize how fortunate some of us are to know the quality of our meds better, especially in cases where it's desperately needed for quality of life.