silver mercury glass - vintage kimiko guardian coasters 
 
popular mid-century mercury glass, trojan soldier coasters {rare vintage set of six}
 
mercury glass also known as “silvered glass” is glass that was blown double walled, then silvered between the layers w. a liquid silvering solution & sealed
 
silvered mercury glass is considered one of the first true "art glass" types, as it was made for display + for its beautiful artistic value rather than its utilitarian use