Islamic gold coin. This gold coin is from the Portalon cave (the entrance cave) to the Cueva Mayor, a cave in the Sierra de Atapuerca foothills near Burgos, Spain. The limestone caves here, containing many prehistoric fossils and artefacts, have been designated a UNESCO World Heritage Site. The caves were also occupied at later times, during the Roman and medieval periods. The gold coins found here date from the 10th to 13th centuries, when the region to the south was under Muslim (Arab) rule.