Hallmark: Birmingham
Sponsors Mark: CR
Stamped: TGGC
UK Size: R 1/2
Head Size: (L) 10 x (W) 9 x (H) 3mm
Shank Width: 1mm
Weight: 2.3g
It is mounted gemstones doublet or triplet. I think it is opal or ammolite but not 100% sure.