Coastal shells found in the highlands have been dated at 5,000 to 9,000 BP. Flakes of obsidian, from Lou island in the Manus group, have been found in the New Hebrides, 2,700 kilometers away. Flakes found in the Matthias Group, 250 kilometers from Lou, are possibly 3,000 years old. Pottery appears to have been traded, among islands and along the coast, for several thousand years. Pottery was also traded from the coast into the eastern highlands; the earliest evidence of this trade found so far dates back 800 years. Much recent archeological work has been focused on a kind of pottery known as Lapitaxc. This was widely traded, and is thought by many prehistorians to be connected with the spread of Austronesian languages. Stone for weapons and tools and salt also appear to have been traded for thousands of years. Trade in fish, sago, pigs, vegetables, salt, shells, pots and feathers was observed at the time of European contact. Much long-distance trade was not direct, but through a chain of societies, leading from the point of supply to the final consumer. Thus highlanders greatly valued sea shells imported from the coast, but did not know that seas existed, or have any idea of the origin of the shells. Trade was of social and political, as well as economic, significance. Trade by barter is still important in the subsistence economy.