According to the seller, their master gemologist says the cavity is inside the diamond, so there is no risk of dirt getting into it.
Video of diamond: http://elements.dharamhk.com/Video/QSBWE019
You can see it pretty clearly in the video, but it is clearly very zoomed in. I get a halo setting for whatever diamond I choose (not sure if it matters here).
This one is at 1.3 carats, excellent cut and is below my 10k budget.
Is it worth it, or should I keep looking?
GIA Report: https://www.gia.edu/report-check?reportno=2277346386