  We consider the production of heavy quark jets at the $Z$-pole at the
next-to-leading order (NLO) using the {\it Cambridge jet-algorithm}. We study
the effects of the quark mass in two- and three-jet observables and the
uncertainty due to unknown higher order corrections as well as due to
fragmentation. We found that the three-jet observable has remarkably small NLO
corrections, which are stable with respect to the change of the renormalization
scale, when expressed in terms of the {\it running quark mass} at the
$m_Z$-scale. The size of the hadronization uncertainty for this observable
remains reasonably small and is very stable with respect to changes in the jet
resolution parameter $y_c$.
