They've been observed in Martian meteorites as well, I mentioned them in an earlier post.

Tectites are an excellent possiblitity, most likely the best answer for their origin. Especially that little hole many seem to display, that is where the paired sphere (they often form dumbell shapes in flight) pulled away.

Remember, volcanic and impact glass is not like window glass. It has the composition of the melt, which could contain just about anything, including metals.

I'm still considering the difference between their surface and subsurface expression, if there is a minute amout of water in the subsurface it's gonna be ice and therefore fairly chemically inert. There may be pH/Eh reactions with minerals in the soil here, don't discount the effects of solar/interstellar radiation either...