Many years ago there was an exhibit at the Los Angeles County Museum of Art (LACMA) of various objects found at Pompeii that had been covered by the ash from Mount Vesuvius. I remember seeing the original of the above mosaic in the exhibit, which looked much better than the illustration above.
According to an article by Mark Cartwright published in 2013:
Mosaics, otherwise known as opus tesellatum, were made with small black, white and coloured squares typically measuring between 0.5 and 1.5 cm but fine details were often rendered using even smaller pieces as little as 1mm in size. These squares (tesserae or tessellae) were cut from materials such as marble, tile, glass, smalto (glass paste), pottery, stone and even shells. A base was first prepared with fresh mortar and the tesserae positioned as close together as possible with any gaps then filled with liquid mortar in a process known as grouting. The whole was then cleaned and polished.
In addition, there were wall paintings from Pompeii, but these dis not impress me greatly. It was as if painting was a kind of poor man’s version of mosaics. What surprised me was that, in so many instances, there were paintings of statues.
There were even some historical mosaics, such as this badly damaged view of Alexander the Great and his army:
In almost every case I have seen, the Roman mosaics were superior to the paintings of the period that I have seen. When one sees the original of one of these mosaics, one is impressed by the vividness of the image and the superiority of the medium. When I see a Pompeii exhibit or attend the Getty Villa, I always end up feeling that, with the end of the Roman Empire, we have lost a great art form.