Ever since the CIA invented crack and introduced it to America in the 1980s, this drug has found its way into a number of plays. Thus it is up to the prop master to figure out how to simulate its use on stage, since smoking real crack is often beyond the budget of most productions.
The most important step is first to research what crack looks like. With the internet, it should not be too hard to find images of the actual drug; relying on pictures from film is less reliable because these are already an interpreted facsimile of the original. If you can get your hands on some actual crack, take some pictures of it before you disappear into a years-long struggle with addiction.
In the film Half Nelson, actor Ryan Gosling smokes crack on-screen. Jeremy Balon, the prop master, explains how they achieved the effect:
What ended up working was an off-white coffee mug that I broke up into about a million little pieces, then dyed in coffee. During the scenes I would use a piece of the broken porcelain that most resembled a ‘rock’ and then set a small ball of tobacco behind it, so that when lit, a very small amount of smoke would come out.
If your theatre prohibits tobacco, you can substitute a small amount of material from an herbal cigarette. If even that is prohibited, you will need to experiment with other materials which will give off a small amount of smoke over a prolonged period of time.
The Prop People Forum has some more helpful suggestions. One user found some white gravel in a parking lot which worked perfectly. In that case, the director did not want to see the crack actually smoking.
Other suggestions which were offered but not tested out included clear rock candy or breaking chalk or sugar cubes into irregular shapes.