What is limestone?
Limestone is a kind of sedimentary stone that is very common all
over the Mediterranean and Europe.
Limestone is made out of the shells of zillions of little tiny sea
snails and creatures like that. These snails and stuff lived in the
sea, billions of years ago, and when they died they fell to
the bottom of the sea and rotted, but their shells, which were made
of calcium like your teeth, did not rot and just stayed there. Pressure
from other shells, and from the water, and from sand being washed
over the shells, squashed them all together into rock.
Many many years later, the sea changed where it was, and all this calcium-rock (limestone) was left on the land where people could quarry it (dig it up).
Limestone is also the main way you get lime, which is one of the things you make cement out of. To get lime, you have to burn limestone in lime kilns, and then what is left is lime. In the Middle Ages, people burned most of the statues of ancient Greece and Rome to turn them into lime.
DK Eyewitness Guides: Building, by Philip Wilkinson (2000).
Geology: A Golden Guide from St. Martin's Press, by Frank Rhodes (2001).