Coke is always cut, that is one thing you can be sure of. Virtually all of it is cut before it even leaves South America. Some of these cuts are inactive like lactose, vitamin b, baking soda, sugar, mannitol (baby laxative) and others active such as ephedrine, caffeine, meth/amphetamine with procaine or lidocaine added to enhance the numbing effect. The active cuts are the problem because they make the coke high feel dirty and leave you with residual stimulation and anxiety.
Only way to be sure that you're blowing the purest coke you can get is to purify it yourself. Anybody with a good amount of coke, some basic chemistry knowledge, anhydrous acetone and muriatic acid can do this in the privacy of their own home.