The material seems to be either marble or granite. Marble or sandstone would most likely be weathered stronger. However granite is rather resilient to weathering. There is some evidence of weathering (rounding, discoloration) - but no major weathering. It is almost certain that the material did not come from the Island, since it evidently is a purely volcanic island - like all the remote islands in the Pacific. They usually are the tops of underwater volcanoes (like Hawaii). This may support the theory that the foot did not originate from the Island, but was built elsewhere and somehow transported to the Island later.