That is known as a mud concretion. They form when leaves, etc. group up thousands of years ago. The ball of crud eventually rolls to a point where it creates a mudball covering the initial sticks or leaves. As time goes on and the mudball rolls around over thousands of yearsthe leaves etc are long gone from decay. Sometimes this leaves a hollow area in the center where water and mineralized solutions have seeped in. They will eventually evaporate out sometimes leaving a hollow mineralized center full of either calcite or quartz.
If you broke it open or cut it in half with a rock saw, you might find it to be full of crystals. You can tell the better ones because they are light. Got one sitting in my backyard that is about 15 inches across and weights about 100 lbs.