Natural-color mica. These sheets make a nostalgic window when laid over a photo (they are transparent with a tan color cast). You can peel off thinner sheets easily with your fingernail (see one of the additional photos; mica is a mineral with crystals that align themselves in flat layers).
It can be cut and curved if heated (it's also a good insulator so is a safe window over/around a candle).
Tear, die-cut, or trim with scissors. Mica can be embossed, stamped, painted, collaged, used as photo transfer canvas, and adhered to virtually any surface.
An old-school electrical insulating material, also used as window material in wood-burning stoves. Also known as "isinglass" (the "surrey with the fringe on top" had isinglass curtains).
1 ounce package (1-2 thick layered pieces, approx 2" x 3": each piece can be peeled apart into approx 15-20 sheets)120912RFnym 130310rwng 140306rft 150705kbt 160908NB 150801sbb 181007KB 181007new 190310SC 220410new