After a long hiatus another implementation of the Weekend Fun puzzles! I've been searching through my pet rocks and I found this one. It is just right I think for asking: What is it and how does it form? I left away scale and other clues. The shown sample has been found in Germany by a friend of mine some years ago. Which isn't really any help because these can be found worldwide as far as I know. Have fun guessing! I'll solve this weekend fun puzzle sunday or monday depending on the number of comments unless someone get's it right first. Good luck!