Iris glass (often called rainbow glass) was developed in the late 19th and early 20th centuries in the Czech region.
Nowadays we like sparkly genuine stones, but in those days a lot of jewellery was made with paste (glass) stones, often backed with foil for extra glow, pinchbeck, marquisite, and rhinestones (named after the Rhine river). It was more affordable, and the stones looked gorgeous in candlelight. Older diamond cuts were faceted to look good in candlelight, which means that they look less sparkly by today’s electric light.