The mud-brick core of the ziggurat was covered with baked bricks.
Each baked bricks measured 31 x 31 x 9 cm.
Some of these bricks were stamped with the name and title of Nabonidus, who rebuilt the ziggurat. These are slightly larger than the bricks Ur-Nammu used to build the ziggurat originally. So even if they are not stamped, we can tell they belong to Nabonidus' rebuilding.