Jeju Stone Park celebrates the culture of stone, which is deeply connected to the volcanic island. It features stones and stone sculptures with different purposes, shapes, and material as well as the science of geology. It is a unique place worth visiting if you come to Jeju, and it might take you any time between 2 hours (if you only want to see the outdoor exhibitions briefly) to a whole day (if you're a stone geek).
Being quite high up in the mountains, the landscape looks dry and grey. I sort of wish the stones were set in a deep forest instead, but I suppose that would be very artificial.