From what I understand, methamphetamine that comes in shards (big crystals) is not only not adulterated, but is also mostly dextro-meth. However, there are impurities that come from manufacture, which are leftovers, what are called 'side reactions' and the product of a sloppy cleanup.
A partial list follows: Toluene, Benzaldehyde, cis-1,2-Dimethyl-3-phenylaziridine, 1-Phenyl-2-propanone/ Amphetamine, N-Ethylamphetamine, N,N-Dimethylamphetamine, Ephedrine/pseudoephedrine, N-Acetylephedrine, 3,4-Diphenyl-3-buten-2-one, N,N-di-(β-Phenylisopropyl)amine, 1,3-Dimethyl-2-phenylnaphthalene, 1-Benzyl-3-methylnaphthalene, Me(α-Me-Ph)amino-1-Ph-2-propanone, Benzoylmethamphetamine, N,N-di-(β-Phenylisopropyl)formamide.
Some of these compounds are in all meth and others are route specific and law enforcement uses them to trace where it was made. A few of these compounds, such as 1,2-Dimethyl-3-phenylaziridine are thought to be responsible for some of the extreme damaging effects of meth.
Any attempts to remove these impurities would likely be futile for someone who is not a chemist.
Having said that, it is found that the big shards are more pure and the crumbs that surround the big shards are more loaded with these impurities making this powdery stuff just downright nasty.
Do what you want with this knowledge, but it seems to me that people are consuming toxic waste.