Home 13. Cut and polished surface of an ovoid pebble of dolerite from the Whin Sill (Northumbrian Coast). The glossy black of the polished surface contrasts with the grey, speckled and worn exterior 13. Cut and polished surface of an ovoid pebble of dolerite from the Whin Sill (Northumbrian Coast). The glossy black of the polished surface contrasts with the grey, speckled and worn exterior Toby Keel August 31, 2018