Stained glass
Stained glass was a type of artwork that was made up of colored glass.
In Equestria, stained glass was an important cultural and historic aspect, and it was often used to denote and depict historic events, ponies and other things important to Equestrian history. In the Hall of stained glass leading up to Canterlot Tower, stained glass depictions of figures such as the Mane Six and Discord were displayed.