Type of Rock

Limestone is a sedimentary rock primarily composed of calcium carbonate (CaCO3) in the form of the mineral calcite. 

What does it look like?

Limestone is typically light-colored or gray and may contain visible fossils. 

How/Where is it made?

The primary mineral in limestone is calcite, the mineral found in shells.

Marine organisms (creatures), like corals, algae, mollusks, scallops, bryozoans, and other creatures with shells, create calcium carbonate shells and skeletons. When they die, their shells accumulate on the ocean floor and over time, the layers of fragments press down on each other, squeeze out water, and recrystallize into limestone. This process is called bioclastic limestone formation. (See picture below)

Does it belong?

Limestone in this amount would normally be found on an Island like Thera (Santorini).