Metallic Foil Coated Four Way Stretch Nylon Spandex. 
80% Nylon, 20% Spandex.
Looks like colored aluminum foil but is stretchy! 

Metallic will rub off if rubbed excessively. Foil will separate from spandex backing if worn too tight. Hand wash inside out in cold water, line dry. Iron inside out on low heat. Do not bleach.