  The 7.6(5) eV nuclear magnetic-dipole transition in a single 229Th3+ ion may
provide the foundation for an optical clock of superb accuracy. A virtual clock
transition composed of stretched states within the 5F5/2 electronic ground
level of both nuclear ground and isomeric manifolds is proposed. It is shown to
offer unprecedented systematic shift suppression, allowing for clock
performance with a total fractional inaccuracy approaching 1 x 10^-19.
