Opaque Neon Yellow YAG.  Yttrium aluminum garnet (YAG, Y3Al5O12) Lab created garnet with a RI of 1.833 and 8.5 hardness, making it suitable for everyday jewelry. YAG is also known for its fabulous dispersion.  Sourced from Kazakhstan.⁠ 

Photo of finished cabochon provided for reference

135.47 grams / 677 carats / 85 x 36 x 10 mm