We study the nucleation of quark matter drops at the center of cold
deleptonized neutron stars. These drops can be made up of unpaired quark
matter or color-superconducting quark matter, depending on
the details of the equation of state for quark and hadronic matter. The
nature of the nucleated phase is relevant in determining the critical
mass $M_{cr}$ of hadronic stars above which a transition to a quark star
(strange or hybrid) is possible. We investigate the dependence of
$M_{cr}$ upon the parameters of the quark model (the Bag constant $B$,
the pairing gap $\Delta$, and the surface tension $\sigma$ of the
quark-hadron interface) for different parametrizations of the hadronic
equation of state. For a large part of the parameter space corresponding
to hybrid stars, the critical mass is very close to (but smaller than)
the maximum mass of hadronic stars, so compatible with a ``mixed''
population of compact stars (pure hadronic up to the critical mass
and hybrid above the critical mass). For very large $B$ the critical
mass is never lower than the maximum mass of hadronic stars, impliying
that quark stars cannot form through the mechanism studied here. The
energy released in the convertion is sufficient for powering a gamma ray
burst.