Iron Ball, Hollow w/ Something Inside

[AmyM]

I was digging in my flowerbed and came upon an iron ball with something inside. I see no seam and there are a few small specs of green that look like the original coating. It is just a little bigger than a golfball. There is what looks to be a faint design and number or word, but I can't make it out. What's a safe way to clean something like this?

 

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[motel6.5]

This is not related but I have heard of a person finding iron balls in Texas the size of large cannon balls or more filled with treasure. Iron balls dated to after the Texas war with Mexico or were somehow connected.