Today, I found this coconut-sized fossil (or just rock) partially imbedded into very brittle (shale-like) cliffside along a hiking trail on Oahu, about 2,000 feet in elevation. The branching pattern along the surface is what caught my attention. At first, I thought it looked like a coconut, but then remembered that coconuts are not actually indigenous to Hawaii. I then press my thumb against the front and a chunk crumbled off, revealing highly defined internal layers. I’m actually a chemist, not a geologist; so I am fascinated, but at a loss. Fossil or not, can anyone help me understand what this might be and how it was formed? Thank you, Richard