Mica is a mineral that cleaves in one direction and can produce then sheets that are translucent, somewhat "see through." See Biotite Mica Mineral Specimens for a picture. Years ago, it was often used in place of "expensive" glass, especially for applications where there would be a lot of heat, such as Lampshades and windows for wood burning stoves.

But my take on mica is that it somewhat fragile and I would expect that a commercial signal mirror, such as the one in Doug's kits, would be more durable and easier to use (and easier to find).