This is a little larger than your average Victorian pools of light pendant. 
2 perfect hemispheres of rock crystal, set back to back as a locket, with a little recess in between where you could add a photo or flower. 
I love them worn just as they are, playing with the light. 
The hinged silver setting has a little thumb node to aid opening.
I would wear this on a slightly longer chain. 
Tested as silver 
Weight approx 11.28g
Total drop approx 29mm
Width approx 23mm
Very good antique condition - no chips to the stones