This is the beach on the tombolo that joins the one-time island of Seongsan Ilchulbong to Jeju. It’s formed from the volcanic rocks eroded from the island in the 5000 years since it was formed. The main part is the dark pebbles (well, it’s too big to be called sand) formed from the rocks. It’s mixed with an array of different sea shells and corals.
I knew it was a tombolo because the noticeboard told me so, and I’ve just looked it up on the internet. A tombolo is a spit of land that forms between an island and a mainland due to the tidal deposition of material. Chesil Beach is one example, joining the Isle of Portland to the Dorset coast.