It is the tallest dome in the world. At nearly 450 feet high, the Dome of St. Peter’s Basilica at the Vatican is awash in color with scenes depicting the 2,000-year-old history of the Catholic church. Near the top, the Latin inscription proclaims, “…upon this rock, I will build my church,” which is ironic, considering that the entire interior dome is decorated in small, colorful stones.

These scenes carefully crafted from stone are so intricate that from below, they appear to be highly detailed paintings, yet they are not. They are mosaics, or more precisely, micromosaics. Large stones that once formed Roman roads are now small stones which, ironically enough, feature in the works of the church the Romans persecuted.

How a Mosaic Begins

Arranging large stones to form a functioning roadway, building or walkway is a civil-engineering technique that dates back to early Mesopotamia, now Iraq, about 5,500 years ago. In fact, the earliest stone road that still exists lies in Gaza and dates to at least 4,500 years ago. It may have been used to transport blocks of limestone that became part of the Egyptian pyramids.

Arranging stone pieces to form mosaic artworks is also ancient in origin and was prevalent throughout the ancient Greek, Roman and even early Mayan periods. The difference is in the size of the stones and how they are arranged.

Mosaic creators differentiate their patterns with color. The colors come from different types of cut stone, glass, ceramic and even enamel. Cut small and usually square, each piece, called a tessera (plural: tesserae), is painstakingly fitted together on a setting bed of plaster or cement, one piece at a time. The work is destined for installation within an outline brushed on a harder, more permanent surface, such as a wall, ceiling, column, fountain or indoor floor. The more elaborate the mosaic, the more prominently it was placed. Mosaics were status symbols when they were new. Today, they are considered national treasures and are not generally licensed to be bought or sold on the open market.

Mosaics Go Micro

When the dome of St. Peter’s Basilica was completed in the early 17th century, it created its own vapor clouds that eventually made it difficult to maintain its original paintings and frescoes. In the late 16th century, under the pontificate of Pope Gregory XIII, each painting and fresco was replaced via a new process that relied on long, thin, colorful segments of highly durable glass (smalti) cut into small, almost microscopic squares (filati) and placed, one by one, to recreate each masterpiece in a mosaic pattern.

The Vatican Mosaic Studio has maintained the intricate micromosaics in museum-quality condition since 1578. The studio continues to recreate paintings and images using a palette of some 28,000 separate smalti filati colors to produce micromosaic art as gifts for heads of state, for restoration projects at the Vatican, and for sale to pilgrims from around the world.

The Grand Tour

Micromosaic art was resurrected in the late 18th century but was channeled into the creation of a more compact array of wearable jewelry and smaller decorative items, rather than large murals intended for public buildings. The artistic technique reemerged at precisely the right time.

From the 17th- to the late-19th century, the Grand Tour was a rite of passage for many privileged sons and daughters of the British upper class, in particular. The canonical Grand Tour itinerary included visits to Paris, Venice, and particularly Rome. The schedule of museum tours, concerts and cultural introductions led to a trade in beautiful, highly detailed micromosaic jewelry depicting many of the sites the young travelers might have visited. Micromosaics became a favorite keepsake of the Grand Tour experience, with each image of tessarae so fine – up to 5,000 small pieces per square inch – that they were regarded as works of art on a smaller scale.

These souvenir pieces weren’t just executed in small tesserae, but also in pietra dura, a phrase that translates as “hard stone.” Not unlike stained-glass art, pietra dura jewelry featured different shades of stone carved and shaved to fit together into delicate images of flowers, animals or historic architecture, usually set against a black background. The pietra dura style began in 16th century Florence using polished stones such as agate, mother of pearl, lapis lazuli, jasper, and other colored, sometimes semi-precious stones, and can almost be compared to marquetry, as each piece is tightly fitted without the use of grout or other cement to secure it.

Another standard Grand Tour souvenir, particularly during the Victorian era, was a parure: a set of intricate matching jewelry, usually consisted of a necklace, earrings, brooch, bracelet and, at times, a tiara, all in micromosaic, and all packaged in a fancy clamshell box.

The Artists Collectors Look For

Collectors are drawn to micromosaic pieces with tessarae so fine, they look almost like paintings from afar. Fortunato Pio Castellani, one of the early micromosaic artisans of the late 18th century, specialized in reproducing images of ancient Etruscan archeological finds. His makers mark is a mirrored “C” in a lozenge shape and also the word “Castellani” inside a raised rectangle.

Collectors also appreciate the works of Giacomo Raffaelli, who helped pioneer early micromosaics when he set up his shop in Rome in 1775. His signature style consisted of setting square tessarae in rows to create an image of limited color and design.

Michelangelo Barberi was known for the immensely fine micromosaics he created for many royal courts and the Tsar of Russia. Domenico Moglia, Antonio Aguatti, Luigi Cavaliere Moglia, Filippo Puglieschi, and Luigi Podio are also among the prominent artists of the late-18th and 19th century whose work is found at auction.

The Golden Age of micromosaics lasted until about 1870, when the tourist trade supplanted the Grand Tour. Micromosaics took on a more noticeably cruder look after that point, with larger stone tessarae replacing the finer artistic approach of the late-18th and early 19th centuries. Such works possess limited collector or auction appeal.

In addition to wearable jewelry, fine micromosaics can be found on elaborate snuff boxes, tabletops, panels and plaques, which were originally sold as souvenirs. The Victoria & Albert Museum’s Arthur and Rosalinde Gilbert Collection is one of the largest collections of micromosaics in the world, named for the collector Sir Arthur Gilbert, who coined the term “micromosaic” in the 1940s to describe the delicate art.