Holding it in hand and rotating it, I looked to see if I could find any remnant of quartz and find none. The base of the groove and the sides of the groove appear to be the same material (sans patina) as the outside of the rock and similar in color to the flat are on the lower right corner of the above photo. I'm no geologist and not sure how probable it is that a band of quartz would occur on the rock all the way around at a uniform depth such that the ablated quartz would leave a groove of consistent depth all the way around the rock. It would seem to me, again not being a geologist, that if this effect was produced by natural removal of quartz, that we would likely not see a uniform depth of the groove since the quartz (none of which remains) would likely not have occurred in a ring-like deposit around the rock without some irregularity of depth at some point in the groove. Am I making sense?
The rock is extremely hard and dense and heavy. I've not wanted to peck-at it to test its hardness in case it is an artifact but having worked with ceramics and stone in construction, I would expect that fracturing performed with metal punch and hammer in my shop would produce a circular pitting such as shown on the edge of the groove as called out above. I do have to admit that it is hard to imagine a tool that existed in stone period that would be hard enough to fracture this hardness of rock in this way. But then I also have no expertise on what methods and materials might have been available.