Between a Rock..
...and a hard place!
The boys from being very small have always been interested in rocks and stones and despite being all grown up now, still bring them home from their travels. This is one such stone...I think if memory serves me right eldest brought it home from New Zealand having picked it up as he navigated the Tongariro Alpine Crossing, which would make it volcanic in origin.
Thanks to ApolloFly for hosting Tiny Tuesday this month.