Round Glass and Silver Locket. The space between the two glass sections allows the storing of something quite thin, such as a photo, or a petal.
Turning the Silver knob that sits directly above the glass (anti clockwise), as far as it will go up the rifled stem, frees the two parts of Silver-framed glass, and they will open apart on the hinge. Note the look of the  pendant open, in the last image below the main image.
The thick glass slabs are of a faintly green hue, a little like the colour of seawater in a rock pool.