http://bentasker.i2p/posts/blog/116-republished-freedom4all/661-the-link-between-cannabis-and-increased-crime-rates.html
Because there are no standards, buyers of Cannabis could easily be paying extra for the ‘privilege’ of smoking glass or boot polish. Cannabis resin (hashish) is often cut with other chemicals in order to artificially increase the weight so that a better profit can be obtain. Cannabis buds are sometimes sprayed with carcinogenic glass for the same reason.