Grainy black-and-white footage of Elizabeth II’s coronation shows the young queen enthroned, amid the panoply of state, before the altar of Westminster Abbey. But a key element of the ceremony, which has been carried out in the same place and according to the same traditions since the thirteenth century, was hidden from view by a large expanse of monochrome institutional carpeting. Beneath that carpet, neglected but not quite forgotten, lay one of the masterpieces of the Middle Ages – a spectacular Cosmati-work floor, designed in an intricate pattern of interlocking spheres and squares, formed from a mosaic of over 80,000 separate pieces of stone.

The great Cosmati pavement had been an integral part of the coronation ceremony since the time of Henry III, when it was created to evoke the harmony of the spheres and the perfection of heaven. Its rich symbolism was still understood in the age of Henry VIII, when Holbein placed his melancholy Ambassadors, who dream of salvation in the world to come, on a Cosmati-work floor just like the one in Westminster Abbey – and in the age of Elizabeth I, when Shakespeare wrote that “the floor of heaven / Is thick inlaid with patterns of bright gold: / There’s not the smallest orb which thou behold’st / But in his motions like an angel sings...”

But by the end of the nineteenth century, the brilliant inlaid floor had been covered over. Only now can it be seen, once more, in all its magnificence. Thanks to a vigorous fundraising campaign, the Abbey has been carrying out long overdue repairs to its delicate fabric. Throughout this summer, work will continue to clean and fix the pavement’s many thousands of mosaic tiles. The restoration is being carried out in open view, so anyone who wants can witness the making good of this fascinating, precious piece of history.

The restoration of its floor also restores a part of Westminster Abbey’s larger symbolic significance. The soaring Gothic arches of the Abbey are a vision in stone of the Heavenly City, leading the eye upwards into space and light. The floor, by contrast, offers a kind of blueprint of the medieval idea of perfection, its geometry intended to mirror the perfect logic of God’s plan. Its mosaic patterns symbolically link heaven and earth.

At its very centre is a perfect orb of richly veined onyx, which itself looks uncannily like an image of the earth circling in space, wrapped in a layer of clouds. There was once an inscription running around the circumference of the floor, now rubbed away by time, but recorded by antiquaries before it disappeared. As well as giving the precise date of the end of the world – we still have another 18,000 years, according to medieval theology – the inscription also alluded to the central orb. “Here is the perfectly rounded sphere which reveals the eternal pattern of the universe.”

The orb of onyx marks the spot where the throne must be placed at every coronation. It is like a prayer set in stone – as if to draw down divine power on the monarch, through the circle of the crown to the beautiful circle at the centre of the floor. To look at it is to feel the tremendous force with which the ancient idea of the divine right of kings was once asserted – an idea that was effectively overthrown, in England at least, with the beheading of Charles I, who like every other English monarch since the time of Henry III had been crowned on this very spot.

The great pavement of the Abbey is a tangible piece of the English past, but it was actually made by Italians. The creation of the floor was a papal favour granted to Henry III, who was determined that Westminster Abbey should match the great French Gothic cathedrals of Chartres and Amiens. The Cosmati were a family of crafstman based in Rome, where they were allowed by papal charter to pillage the monuments of antiquity for the highly prized stones that they used in their work. The Romans themselves had pillaged Greece and Egypt, so when the Cosmati craftsmen came to London in 1267 to make Henry III’s pavement, they brought with them fragments of stone from all over the known world – purple porphyry from Egypt, chosen for its imperial associations; green porphyry from Greece, petrus Christi, so called because within its crystalline form hundreds of tiny cross-like forms are clustered; shining pieces of cobalt, turquoise and red glass, probably made by Venetian glassblowers using a technique that had been imported from the Islamic East.

There are many Cosmati and Cosmatesque pavements in Rome and the papal states, including a particularly splendid example in the Sistine Chapel. Michelangelo, who looked down at it every day from his scaffold while painting the Sistine ceiling, may have had the splendid image of its speckled mosaic in mind when he wrote a famous poem lamenting the difficulty of his task: “My beard toward heaven, I feel the back of my brain / Upon my neck, I grow the breast of a Harpy;/ My brush, above my face continually / Makes it a splendid floor by dripping down”.

The Cosmati pavement in Westminster Abbey is by far the most magnificent example of such work to be found north of the Alps. It is indeed a “splendid floor” – and well worth visiting.