If you buy "glass" on the street, then it's probably of a fairly high purity - simply put, "glass" is damn hard to cut (that is, the glass like shards) .. the pink stuff that crumbles to a powder is a lot easier to cut .. still, it all smokes..
Generally methamphetamine isn't cut - yeah, I know, a lot of people will argue... but most meth, is, in fact, NOT CUT ... the problem is so much of it is of such shit quality..