For a rough & ready version of this.. have you got legendary items 4.x?or higher (4.1-4.3 or V5) Because that has the meshes & textures to make pretty much what you want.
Try putting this on the end of your main items.CFG
[Back Gem]
mesh=teargem.mesh
material=NecklaceMatSapphire
bone=Main
offset_f=0 2 -6.1
rotation_f=0 0 0
offset_c=0 2 -6.1
rotation_c=0 0 0
A back mane will obscure it somewhat. But it looks almost exactly how you describe.