These mosaics can be found on the celing of the Mausoleum of Galla Placidia, in Ravenna, Italy. The building was constructed in 430 AD and was supposed to serve as the final resting place of Galla Placidia, daughter of Roman Emperor Theodosius. The mausoleum also has prominent mosaics featuring themes from Christianity, and the ones pictured are on less important parts of the walls and ceilings but they are the ones I chose to feature here because I am a fan of geometric designs, especially in terms of mosaics and I find these simply breathtaking.
As you may have guessed from the date and the mention of Christian imagery, these mosaics date back to the Byzantine Period. By then, mosaic is not really a new form of art, but the innovation during the Byzantine era comes with the introduction of glass thessera (mosaic cubes) as opposed to archaic stone ones. Another interesting point to note is that the gold parts of the mosaics are made of glass cube that were actually gilded, so this is not just the iridescent effect of the glass. You can find a lot of this trend illustrated in the auras of people represented on Byzantine mosaics, such as the mosaics of Emperor Justinian.