u/ProfessorPitiful5680

I was gifted this ring by a customer today, and have no clue what the stone could be! The inside of the band says GTR 10k on the inside. The stone itself appears black and opaque in regular light and surface is shiny, but when put up to a flashlight, the light shines through in a range of blues and greens. My camera seems to capture is brighter than I see it with my own eye, but am super curious! I don’t really care if there’s any value (i doubt there is lol)