If you have an old stove with windows to the fire then this may well be made of mica. Mica is very thin, opaque and sometimes yellowy
On old stoves the window is sometimes made of mica. Mica is silicated mineral that is mined from the earth. Mica breaks off into sheets because of it's chemical structure. These mica sheets are opaque and heat resistant and when used in stoves are about 1mm or less thick. This makes mica suitable as a replacement for glass in stove windows.
Mica was used in older stoves and the window panes tended to be very small. This is because mica sheets are relatively small and delicate - you could poke a hole through a large mica sheet fairly easily with a pen. Stove manufacturers stopped using mica With the introduction of ceramic heat resistant 'glass'. Mica is available in various sized strips.