The shape of this unguentarium is characteristic of Roman Egypt: the thick walls creating a durable container with a very small capacity.
Unguentaria were amongst the most common objects of Roman blown glass: produced in large numbers, they were items of every day use for keeping expensive unguents and cosmetic oils. By the 1st century AD, the technique of glass-blowing had revolutionised the art of glass-making, allowing for the production of small medicine, incense, and perfume containers in new forms. The small body and mouth allowed the user carefully to pour and control the amount of liquid dispensed, and glass was the material of choice for storing the oils because it was not porous. These small glass (or ceramic) bottles are found frequently at Hellenistic and Roman sites, especially in cemeteries, and the perfumes which filled them would have been gathered from all corners of the expansive Roman Empire.
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.
Similar item: Ancient Glass at the Newark Museum (USA), item 50.1836.