Famous green welded ignimbrite on the NE side of Berufjordur, erupted from a nearby silicic caldera in the Miocene.
See also:
https://blogs.agu.org/mountainbeltway/2016/10/06/oddball-icelandic-rocks-part-green-ignimbrite/
https://viewer.gigamacro.com/view/Yf6NKFLAK3JZNzOx?x1=12750.00&y1=-10526.00&res1=36.49&rot1=0.00