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First, I think this one is a hoax. If they were serious about giving us information, they would put the dust from the center of one of these things in a scanning electron microscope with x-ray spectroscopy and tell us what elements the dust contained. They would also give us the exact composition and crystal structure of the "nickel-steel alloy." Furthermore, I don't believe that these crude pieces are as balanced as one of the referenced sites claims.

However, as a hoax, someone has put some thought into this one or got lucky. Most meteorites are made of iron and nickel. One of the referenced sites suggested that these things were formed under zero-gravity conditions. If little bits of iron-nickel molten metal were thrown off by some astronomical event and they contained a fair amount of absorbed gas, I could see them maybe ending up in the form mentioned. As a floating puddle of molten iron-nickel in space, they might tend to form little balls. A sphere has the lowest surface area to volume ration, so forming a sphere is a way for the molten metal to minimize surface energy. If the outside quenched and solidified while a large amount of gas was still in the metal, the inside might form pockets of gas and be spongy. With time and enough heat, hydrogen gas might break down into hydrogen atoms, diffuse through the metal, reform on the outside surface, and leave as a gas. In refineries, high temperature hydrogen diffuses into carbon steel under certain conditions, so the process could be the same. Eventually, they fell to earth as part of a meteorite.

I don't have an explanation for the ring around the equator of these things except that I still think they are a hoax.

Bill

As a floating puddle of molten iron-nickel in space, they might tend to form little balls. A sphere has the lowest surface area to volume ration, so forming a sphere is a way for the molten metal to minimize surface energy. If the outside quenched and solidified while a large amount of gas was still in the metal, the inside might form pockets of gas and be spongy.

Reminds me of Prince Rupert's drops... Molten glass is allowed to drop into a bucket of cold water, instantly freezing the outside surface into a teardrop shape, then the hot glass inside cools, contracts and adheres to the sides leaving a near-perfect vacuum void in the middle. I wonder if spherical ones have been done in micro-gravity environments.

I don't have an explanation for the ring around the equator of these things

Stretch marks.

Your explanation sounds very plausable. Note the following from the site with the ancient musket balls, uh, I mean molten cast off material.

South African miners have found hundreds of metallic spheres, at least one of which has three parallel grooves running around its equator.

I take this to mean one or two picked up some other impurity and as it rotated through space aquired the rings before cooling.

I don't have an explanation for the ring around the equator of these things

If they were formed in the manner you describe, they'd likely have some net residual spin to their momentum, and that would cause lighter impurities to migrate to the "equator" (relative to the "poles" of the axis of rotation) via centrifugal forces.