Detail of a Roman mosaic of a female head wrapped by two snakes found inside the Roman Terraced Houses, UNESCO World Heritage Archaeological Site, Ephesus. The two sankes are twined through the curly hair of the startled looking woman. The snake heads are facing each other and appear to be hissing aggressively. The tails of the two snakes are knotted below the chin of the woman.
The following information is credited to Wikipedia:
Small pieces, normally roughly quadratic, of stone or glass of different colors, known as tesserae, (diminutive tessellae), are used to create a pattern or picture. By 200 BCE cut stone tesserae were being used in Ancient Roman decorative mosaic panels and floor mosaics. Marble or limestone were cut into small cubes and arranged into representational designs and geometric patterns.
Most recorded names of Roman mosaic workers are Greek, suggesting they dominated high quality work across the empire; no doubt most ordinary craftsmen were slaves. Splendid mosaic floors are found in Roman villas across North Africa.
Often small panels called emblemata were inserted into walls or as the highlights of larger floor-mosaics in coarser work. The normal technique was opus tessellatum, using larger tesserae, which was laid on site.