Cavalier King Charles spaniel necklace, made with antique button

Bought this at an art gallery a few years ago. The necklace is not marked as to whether it is sterling or not but it is silver in color and has been in my closet a long time without tarnishing. The button is silver toned with some gold highlights. I am not sure if it’s sterling silver that is tarnished or if it is of another type of metal. Chain is 18 inches long and is adjustable.