I have been having these things show up in my Zoysia yard for a few seasons now. This is my personal lawn. At first, I thought they were rocks. I went & kicked them hard with my steel-toe boots and about broke my toes. They are EXTREMELY hard and when you finally bust them, they have a dark spore dust that emits from them. Also, as they progress, they eventually kinda bloom out like a serated 8-10 point star pattern and they look like dark brown bumps inside. I can't seem to find out what they are but I suspect some fungus...as I believe that dust to be spore-like. I would go get a picture, but it is dark now and I might try tomorrow or get a light for the camera. Any guesses? or links to pictures of guesses? My toes are still in pain. Thanks!