Vintage with tags necklace Made in the Republic of Ireland.  Stewart & Co.  A little about Stewart & Co. see below.  Not sure of the metal, see the markings on the back of the green pendant.  I believe the stone is some kind of green marble??  but can't be totally sure.  I love the modern design and assume this piece is mid century.  Stewart & Co was an upscale department store and sold luxury items. Will need to polish if silver, I didn't try because I did not want to disrupt the paper tags.   

Stewart's Department Store, a major retail player in Baltimore, was founded in 1901 when Louis Stewart acquired Posner's Department Store on Howard and Lexington Streets. Stewart's later expanded to suburban locations in the 1950s, eventually closing its downtown store in 1979 and its remaining suburban locations in 1983. The store was known for its open layout, enticing displays, and upscale merchandise. 
Will ship USPS Priority Mail