The lead basket, the dangerous nature of it, the burns, the "gaze" (a line of sight effect), and losing its potency does suggest a piece of highly radioactive material. Radiation does weaken stone (via embrittlement or creep) and tends to have different effects on different materials. The interesting question would be, where did they get it -- given that a highly radioactive material would also have a very short half-life, so a natural occurance is hard to imagine.

On the other hand, there is of course that natural uranium nuclear reactor in Africa, as well as some descriptions in really old scripts like, if I recall correctly, in Mahabharata (and the Bible), that are awfully reminiscent of nuclear weaponry. So maybe this is simply another subtle sign that we had already discovered nuclear energy once - and used it to blast ourselves back to into the Bronze age.