One of the most dangerous drugs available to trolls, crystal slam is first mentioned in Snuff. It looks like stone dust, but appears to have little red and blue flecks in it when held to the light. Its manufacture is very difficult, partly because of the unmistakable smell and sparks produced when boiling it. There is a large clue in Snuff that the manufacture of this lethal drug may be expedited by using crushed and powdered unggue pots stolen from the goblins - another reason for persecuting and enslaving them.
As Sam Vimes explains, the first time a troll tries to use it, it makes him think he can walk through walls - and he invariably does. After a few more times, he drops dead. As a result, Diamond King of Trolls has banned it in Uberwald and all other troll cities (including, of course, Ankh-Morpork). Attempting to manufacture or sell it is a hanging offence, and evidence of manufacture will be rewarded with a very large bounty.
It was extremely unfortunate then that crystal slam together with a number of other troll drugs (including Slab, Slice, Slap, and Slunkie) were what Gravid Rust was smuggling (along with tobacco) into Ankh-Morpork.
It has a certain resemblance to the Roundworld drug Crystal Meth.