It helped eliminate dependence on seasonal availability of food, and made it possible to transport food over large distances. However, salt was often difficult to obtain, so it was a highly valued trade item, and was considered a form of currency by certain people. As the human diet moved away from salt-rich game to grains, more salt was needed. Surface salt is relatively rare and mining was difficult and so, as civilisation spread, it became a precious commodity and trading routes were established all around the world. Recorded history also soundly refutes the myth that salt was more valuable than gold. cites Venetian trade documents from the height of the salt trade in 1590 that establish the value of 1 ton of salt as 33 gold ducats.