This is a polished green opal heart. Green opal is a variety of common opal that gets its color from iron. 
Green opal is found in various locations, including Australia, Madagascar, Mali, Peru, Serbia, and South Africa. 
It is considered a cleansing and rejuvenating stone, associated with heart energy and emotional healing. 
The stone is believed to help with emotional heartache, stress, and anxiety. 
It is often used in self-care practices to promote a sense of comfort and well-being