Many people intentionally lace marijuana with pesticides, formaldehyde, glass, sprays, and other chemicals to make the pot potent and heavy. That said, you can experiment with the following things if you suspect your stash is impure: - Try rolling a blunt with pure weed, and you will see the trichomes stay in the roll and stick to your hands. But if the cannabis is contaminated, trichomes become powdery (like dust) that fall off during the rolling process. - If you rub a fresh bud on a CD, it won't leave any scratches. Buds contaminated with glass may leave scratches on the surface.  - Another thing is to check the color and smell of weed. Pure and fresh marijuana smells earthy (Petrichor: like fresh soil after rain). Plus, the color of the weed tends to be greenish instead of black or grey.