have one like it,very hard and tough.Managed to cut it;took a while slow going but solid thru.Looks like Ceramic.kinda like they use for sharping knives.
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Hi,
I lived in Kenya for a few years and quite often guys used to bring me stone the same as this, always said they dug them up and always roughly the same size… I go with theguy who reckons alien eggs! i never managed to find out what they were! 
Hi I am contacting you because I believe what you have are Stones that grow inside the heads of the white drum fish. I live very close to lake Erie and am an avid fisherman as well as a sporting goods supplier and gemologist. I learned about them when searching the beach for Nephrite. I kept finding very similar semi rounded stones that were almost ceramic in appearance. I was told it was ivory… it is not. It is in fact closer to a calcareous conchreation( forgive my spelling on that one). Is the area they were mined from previously ever under water? Ie: Ancient lake or seabed? If so you may have what we refer to in our area of the world is called a DrumS Stone.
On second thought, I didn’t look at the first pics of the perfectly round balls. They are most likely Sling shot load… if found in the same area by several miners, there may have been a roman legion or other military force that used slings at the time in the area. Check any museum that could help you identify it. It is absolutely not a drum stone in the first pics( they would have a domed top and flatter bottom).