Aboriginal group exchanged natural resources, such as ochres, and tools, such as stone axes and boomerangs, thus creating extensive trading networks. Goods travelled hundreds of kilometres from their original source. For example, boomerangs made in Central Australians would find their way to Arnhem Land and the surrounding islands. Didgeridoos from Arnhem Land would find their way down to Central Australia. Pearl shells from the Kimberley were traded through Central Australia down into South Australia. Trading networks were frequently incorporated into formal exchange systems. Large, gatherings of people came together for "exchange ceremonies" where regional specialities were traded. Ritual paraphernalia, sacred ceremonial objects , song verses and dance styles were also passed on from one group to the next at such gatherings.