Glass

Glass is an allowed technology.
Glass is a transparent substance. It is used in a variety of ways, including producing windows for buildings and eye glasses for people that have vision problems but aren't capable of having the problems fixed via magic.
Only a few specific uses of glass are allowed, but those uses are expensive and thus rare. Glass windows are only used by richer people (greased paper and flattened animal horn windows being more common).
Since glass was the only material that was known to properly handle the optics eye glasses needed, glass was added as an allowed technology in 385 alongside eye glasses. As glass was an allowed technology, a few powerful people added it as material use in the other unnecessary cases.