I still have not found any marbles to test my new shelf, however, I did find this:
That is a ball bearing, which is very like a marble but heavier. It is an odd household where one cannot find a marble, but a ball bearing is always close to hand. I am not going to try to explain that.
Anyway, I put the ball bearing onto the shelf, and it sat there obediently, not rolling away. I think that cinches it.
Just for grins, I took an extreme closeup of the ball bearing.
Very M.C. Esher-ish, don’t you think?