Cesium Rocks are a resource in the game which can be quite hard to find and obtain.
For every 20 Cesium Rocks you refine, you get 1 Cesium Block back. To refine rocks, you need them to be in a warehouse.
Here is a list of the location where Cesium Rocks can be found:
Cesium Blocks are the result of 20 refined Cesium Rocks.