What are the hollow ball-like objects in sedimentary rocks?

Geodes. They can be found in some igneous rocks as well. They start as a hollow cavity created by a gas bubble or decaying organism. Mineral rich water seeps into the cavity and crystals form from the outside toward the inside.

The hollow ball-like objects that are found in sedimentary are 'Geodes'.

Explanation:

Some sedimentary rocks have a hollow, roughly spherical cavity right in the middle, making the rock look like a coconut that is cut in two halves. The inner surface of a Geode is covered with small non-uniform crystals. The color of these crystals depends on what minerals are the crystals formed of. The inner surface of Geodes is usually made of minerals like jasper, quartz, celestite, agate, etc.