This rounded object, presumably either a column drum or base, was partially roughed-out while still attached to the rock-face. This is a common technique for rounded elements, especially column shafts, which enabled the quarrymen to identifying any faults in the stone before the whole object was quarried. In this case a crack in the stone - visible at the bottom of this image - did reveal itself and led to the halting of work.
During the roughing-out process a line was carved across the top surface - marked here with the chisel which has been placed next to it. This seems to mark out the centre line of the circle and probable gives the required diameter of the object.
This object is from one of the more than seventy individual extraction points which constitute the so-called City Quarries at Aphrodisias, located roughly 2 km north of the city. White and grey marbles were quarried in this area.