So I did a little followup research and found this site:

Screen cap of geigercheck.com website

geigercheck.com

and… well, today I got a package in the mail:

Padded envelope with label: 'RADIOACTIVE — LSAI'

Just a harmless package

Inside was a little rock:

A small piece of rock in a little zip baggie

Autunite piece in visible light

and a small UV light that makes the rock do this:

Same piece of rock, lit with UV, glowing green

Autunite piece in UV light

And if I put the little rock close to the Nuclear Decay module’s Geiger tube, it does this:

(The envelope output is going to a VCA, with a VCO on the signal input; there’s a low frequency ramp controlling the VCO pitch.)

And yes, this stuff is safe, if you handle and store it correctly.