indy’s most believable supernatural artifact

I think any object that shows any demonstrable supernatural effect is equal to any other.

I assumed the rocks were just superstitious hokum that the villagers believed, and with their "God" gone, their belief that they could survive the random misery (like the drought) was gone, but once the stones spontaneously became red hot and then the one stone immediately cooled before Indy touched it tells us the stones are supernatural.

So if the stones were supernatural, they were no less so believable to me at least than the Grail or the Ark.

I'd have to say the Ark.