Labradorite, is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect if you tilt the polished side in light. 

Not all labradorites exhibit labradorescence, they might not have the correct composition, but cooled too quickly during formation.  Some gemstone varieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.