This is one of the most sought-after cactus species. It is endemic to Mexico.
Slow-growing, this one is on a graft to speed up its growth but it is still only 30mm across.
These miniature species are often worth a close look.
Olympus EM-1, (aperture priority), Olympus 4/3 50mm f2 macro, 1/250 at f11 ISO 800, hand-held.
The stereo is crosseye.