On: Teams in Vienna and Beijing have built the first thorium nuclear clocks

[fsh]

The nuclear clock uses 229Th though which has an 8000 year half life and does not occur in nature at all.

[adrian_b → fsh]

Yes, it is produced artificially in nuclear reactors, thus it is much more expensive than any natural isotope.

The amount used in a nuclear clock would be very small, but even so, its availability would be a problem.