Unguentaria were small perfume or cosmetics bottles made of blown glass. They were extremely popular throughout the Roman Empire, since they contained perfume and oil, considered precious at the time and often used both in private life and public ceremonies. This type of vessels was probably used in funerary and burial rituals, hence their frequent occurrence in archaeological excavations of ancient cemeteries. The iridescence on ancient Roman glass was unintentional, and was caused by weathering on its surface. The extent to which a glass object weathers depends mainly on the burial conditions; however, the humidity, heat, and type of soil in which the glass was buried also all affect its preservation. In this case, the glass has preserved its original translucency.
To find out more about Roman glass please see our relevant blog post: Collecting Roman Glass.