It is a dusty rune. That should have been their first clue that it was dangerous.
Dustwood is technically not a wood from a specific tree. It is an created fuel for fires.
Starting with the sawdust from a sawmill, adding a mixture of wax or pitch, and a few other things, the mixture is pounded or pressed toghether tightly. It is normally smashed into a round container, so when it is slipped out, it looks like a small perfectly round "log". (actually a lathed log, but you get the idea). It can either be used as a log or slices of a log can be used for a fire.