  The recent claim of superconductivity above room temperature in
Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O with 0.9 < $x$ < 1 (referred to as LK-99) has
sparked considerable interest. To minimize the influence of structural defects
and impurity phases on the physical properties, we have synthesized phase-pure
single crystals with $x \sim 1$. We find that the crystals are highly
insulating and optically transparent. X-ray analysis reveals an uneven
distribution of the substituted Cu throughout the sample. Temperature ($T$)
dependent magnetization measurements for $ 2 \leq T \leq 800$ K reveal the
diamagnetic response characteristic of a non-magnetic insulator, as well as a
small ferromagnetic component, possibly originating from frustrated exchange
interactions in Cu-rich clusters in the Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O
structure. No anomalies indicative of phase transitions are observed. We
therefore rule out the presence of superconductivity in Pb$_{9}$Cu(PO$_4$)$_6$O
crystals, and provide some considerations on the origin of anomalies previously
reported in experiments on polycrystalline specimen.
