This is a  beautiful  hand faceted Lambina Opal  set in a Sterling Silver Pendant.  This stone has brilliant red and blue flashes.  The stone is ~7.5 mm.  The pendant is packaged with a 18" 3 mm Figaro Sterling Silver Chain.   Read about the Lambina opal fields here. --  https://opalshop.com.au/australian-opal-fields/lambina-opalfield/