  We describe a novel approach to directly measure the energy of the narrow,
low-lying isomeric state in $^{229}$Th. Since nuclear transitions are far less
sensitive to environmental conditions than atomic transitions, we argue that
the $^{229}$Th optical nuclear transition may be driven inside a host crystal
with a high transition Q. This technique might also allow for the construction
of a solid-state optical frequency reference that surpasses the precision of
current optical clocks, as well as improved limits on the variability of
fundamental constants. Based on analysis of the crystal lattice environment, we
argue that a precision of $3\times10^{-17}<\Delta f/f <1\times10^{-15}$ after 1
s of photon collection may be achieved with a systematic-limited accuracy of
$\Delta f/f \sim 2 \times 10^{-16}$. Improvement by $10^2-10^3$ of the
constraints on the variability of several important fundamental constants also
appears possible.
