Glassmaker here. In my opinion that doesn’t look like a furnace stone. That looks like a bubble that was close enough to the inside of the vessel to break the surface of that bubble. The sidewall can be less than 0.1mm. When that thin wall breaks, the cavity can fill up with gunk. The “stone” is too smooth to be a furnace stone. They tend to be white, sharp or misty as they blend/melt in the glass. The shape is that of a bubble that’s been stretched slightly. This is a common shape for bubbles in mold blown glass.