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.