Apart from beads and seals, hematite was consistently used in Mesopotamia for weights from the late third millennium BC. It is a hard stone which wears well, and it would be obvious if it had been tampered with. Hematite is an iron ore, which is widely found in Syria and Turkey and must have been imported into Mesopotamia where stone of this kind is not available. Weights in this shape were introduced during the Old Babylonian period. These examples range from a full mina (about 500 grams) down to five shekels (about 40 grams).Certain accounting techniques were invented for the trade of bulk items. A system of weights and measures was adopted, partly so that payments to dependent workers on the great palace or temple estates could be reckoned, and also in order to calculate the value of precious objects. Although all the administrators of city states in southern Mesopotamia used the cuneiform writing system, they used various methods of weighing and measuring. With the formation of the empires of Agade and Ur at the end of the third millennium BC, all the cities were united under one king. Attempts were made to reorganise the administration and introduce standardization which must have improved communication and centralized control.