Alan Graham says:
I was digging holes in the yard to plant bamboo when I hit something very hard and big. My two foot wide hole became four feet just to get it out. About the size of a basketball, but was almost impossible to lift. I sprayed it down with water and found some rust colors and assumed it had some iron in it. Magnet proved that assumption, but I was curious as to what type of rock this might be as it has all sorts of interesting markings as well.Mystery rock found in back yard
BTW...I found another rock [shown above], I'm assuming a part of this one, next to it and included photos of it as well.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects