Nut?

I am in Hawaii with the family and I keep seeing "new" things growing or laying on the ground-things that just don't grow in Pacifica. I have been carrying this around for two days now and Jack, my oldest son, pried it open a bit and I was able to get a better look at what it looks like on the inside. I am not sure what the heck it is or what it could be if planted in the dirt but it is a very interesting object to photograph. For those of you who want to know what I did to it..I dropped it into black and white and then used a texture overlay to highlight its complex structure and boosted the contrast a bit..