As you can see, something interputted the agate-forming process, and the agate were left incomplete and hollow. That is also common with Tampa Bay Agatized Corals as shown below.

    Tampa Bay Agatized Corals are banded agate variety even the bands are usually not visible (tend to be more visible when fluorescent or weathered). The picture is a rare example of Tampa Bay Agatized Coral which bands are easily visible. If given time and materials,  this specimen would have kept building up until it's filled up.