Manganese dendrites are not especially rare but they are hard to find in Scotland. We have come across it once before in some old quartzite a friend split open with a pick.
This piece came from the Southern Uplands of Scotland, found on top of a rubble pile. It is a limestone rock with black dendrites made by manganese seeping into or onto the rock over time. 
This rock was found in June 2024. There is a video on TikTok of uncovering it.