Yes all heroin is cut to some degree,usually at the actual heroin lab site first, a very small cut off several % adds up to extra kilos when dealing with 100,s and 1000,s of kilos and the dope will still be/or can be 90%+ pure after cutting it a few % at the lab site. Even the "raw" people talk about getting is cut, even if it's rocks or a big chunk of heroin, it is easy to cut it a bit,and then repress it into what looks like dope that came off a kilogram.
But some places , like NJ lately or in the past few years , have had the purest heroin in the nation according to the DEA, hospitals, Police,etc..with street bags being 70%-80% on average, and also the DEA or police in NJ have seized a few street bags with a purity of aboyt 90%!! That is more the exception, and every city seems to have different cuts added by the dealers, not to mention the cuts in the heroin already. In Chicago for example, Dormin- diphenhydramine aka Benadryl in podwer form is added to the heroin as the main cut in Chicago.
It is common to have heroin cut with anything from powdered milk sugar, quinine, barbituates(mainly in SEA heroin, where they also cut the dope with caffeine powder.SEA heroin is almost never found in the US anymore). Mexican tar is sometimes cut with black shoe polish in the past I have heard,and various B vitamins are often used in powdered heroin on the east coast, Colombian heroin is basicly the only type there.
So in other words the dope may be of high purity, but it still has cut in it, you have to hope your source of heroin cuts the product with some water soluble Vita-Blend B vitamins or some Dormin aka Benadryl powder, and not some other terrible or poisonous cut. Heroin is a gamble on what the street dealers cut it with, but unless they are just rip off artists or junkies, most dealers will sell dope cut with non-harmfull cuts(water soluble viatmins or so on..)..