The stone got me on this one. 
A fiery hessonite garnet, cut with a facetted pavilion (back) and smooth crown (front) which gives an almost jelly like effect. 
 
This was originally a stick pin, likely mid century, and possibly American, The engraved detailing in the surround contrasts the stone perfectly. 
 
We have converted it, adding a recycled gold 6mm jump ring, 
 
Stamped 14WX and tested as 14ct gold.
Weight approx 1.24g
Total drop approx 19mm
Width approx 11mm
Excellent vintage condition