A volcanic rock formed of vitreous substances, obsidian was collected in the vicinity of volcanoes by the Romans to make various types of objects. Roman naturalists had studied the affinities between this mineral and those of glass. The stone's reflective properties made it a prized material for making mirrors. To compensate for its dark reflecting surface, mirrors made of obsidian were placed so as to receive natural or artificial light and so enable the reflection of shapes and forms.