FOR THE PAST 750 years, a 25-foot-square mosaic in front of the High Altar of Westminster Abbey has been the stage for the coronation of British royalty. Known as the Cosmati Pavement, after a Roman family specializing in this type of decorative stonework, it was laid in 1268 on the order of King Henry III. Under the gentle light of suspended brass chandeliers, the colorful tapestry created from more than 30,000 pieces of marble, glass, metal, and gemstones glows and dances. Every monarch from King Edward I in 1274 to King Charles III in 2023 has been crowned on this flooring. And while the mosaic’s gleaming pieces of black onyx, purple porphyry, and green serpentine immediately draw the eye, the stone into which the vibrant circles, squares, stars, and spirals…