According to Kristján Jónasson, curator at the Icelandic Institute of Natural History, this is a fragment of gneiss, an metamorphic rock, most likely Precambrian in age (at least 541 million years old!). It was probably transported from eastern Greenland, as glacial ice rafted debris across the ocean floor. During explosive eruptions from Surtsey in 1963-1964 it was incorporated into the tephra sequence. The ancient rock is now captured in the very young hydrothermally altered, dark grey, lapilli tuff. Simon Prause found this foliated clast was found in core section D-93Z-2 at 275 m in the inclined drill hole.