What is definition of this strange old material ?

Chrono-Shop, your seller will try to explain you...

Sometimes called "cristals plastics" or "acrylics cristals", PLEXYGLAS it'a trademark used for a light, transparent, weather-resistant thermoplastic.

This trademark also occurs in many instances lowercased and spelled plexiglass"The display pieces are double sealed inside plexiglass cases" (Los Angeles Times).

More confortable than sapphirs cristals, they can be repolished for remove scratches or digs.

We built a special collectible list about references cristals distributed in tropics (sample cristals) and cyclopes (cristals with magnifying) :