A long time ago the British government decided to strengthen the marine defenses off the coast of the UK mainland. To do this they used low-cost material which at the time was larvikite - a mineral known for being the sort of less flashy younger brother of labradorite. 
As a result of the government's actions we can sometimes find this material scattered in hillside paths around coastal areas and washed up on the shore. To be clear this piece came from a hill in Dumfriesshire with little to no explanation as to how it got there. We were baffled over this from 2020 right up until we saw a recent Tiktok video explaining it.
So this piece is tumbled, you can find rough pieces in the store using the search function, too.