Apparently this seems to be a phenomenon called "triboluminescence", which is light emitted when certain minerals are rubbed or fractured. Material scientists still don't know much about how/why it works. Not usually seen in sharpening stones, but it attests to the quality of the synthetic ruby grit!
Wint-o-green Lifesavers are one of the more commonly known examples of the phenomenon, which is made so visible because wintergreen oil is reactive to the ultraviolet light given off by the sugar crystals fracturing, which makes the flash visible. The oldest known reference to triboluminescence is attributed to Sir Francis Bacon, who referenced it as a well known occurrence when scraping or nipping the loaves that sugar was made into back then. The large crystals would give off a small visible flash when done in dim light. What's interesting in this case is that it's so strong you can see it even in fairly bright light and, from what I've found on it so far, not all ruby does it? Places that describe it as doing so only say that it "may". Pretty remarkable!
Saw this on my bull thistle american scythe stone sharpening my opinel number 12 in low light today, super crazy! Awesome stone by the way, definitely my main stone for almost everything now, I'm a big fan of a course stone followed by stropping for my working knives.