Construction 1152 - 1363 (211 years)
Largest baptistry in Italy
The portal, facing the facade of the cathedral, is flanked by two classical columns...
Photographs below of the left column demonstrate what sculpted marble exposed to the elements looks like after close to 1000 years.
I am fairly sure I saw a marble quarry from the highway on the way to Pisa, but I am not sure which one. (I made a no photos from the bus rule fairly quickly, because it was too frustrating in the main.) Anyway, check out this link about mining marble at Pietrasanta.
Taking marble from the ground is lot a work:
During the Renaissance, marble was quarried by inserting wooden pegs into naturally occuring cracks in the rock, then pouring water onto the pegs to make them swell. Eventually the rock would split, liberating a piece of marble.
The principal tool of modern quarrying is a wire cable 1cm in diameter, fitted at 5cm intervals with diamond-studded collars. Holes are drilled in the mountain, the cable is threaded through the holes to form a loop, and the loop is driven at high speed by an electric motor.
In this quarry the marble is extracted in rectangular blocks measuring 8' x 8' x 16'. Once the sides and back of a block have been separated from the mountain using the wire cable, the bottom is undercut from the front using a chain saw that translates along a horizontal rail.
If you want to go and look at the famous leaning tower, mi raccomondo you visit at 3am or some other time when the seemingly endless row of tawdry souvenir stalls are closed. (It must be close to a kilometre long and is L-shaped.) This does offer the disadvantage of probably missing a chance to buy a cut-price Rolex watch, but will greatly improve the aesthetics of the location.
It is probable this column is made of travertine, which is technically a type of limestone, not marble. Travertine is formed through the accumulation of calcite from hot springs. Typically, hot water passes through limestone beds and takes the calcium from the limestone into suspension and takes that solution to the surface where the water evaporates and leaves the calcium crystals in layers on the surface. It contains lots of holes that were formed from water flowing through the stone.
Travertine is not of sufficient quality for statuary, even if it will take a very high polish. This explains the prominence of the holes, but only in combination with pollution and weather, particularly water. If acid rain still exists and even fruit juice can "damage" a marble benchtop, then the impact of nature and time on the design has been exaggerated. It is fascinating how the result has integrated with and responds to the carving.
So I am not a geologist, results are not scientifc, sue me.
PS Despite appearances, it still feels absurdly soft and smooth to the touch.