I always wonder if BLUE garnet is sitting among Spinel.
Refractive index (R.I) of garnet family varies from 1.69 to 1.89.
More specifically R.I of Grossular garnet is 1.69 to 1.73, within
the R.I of Spinel which is 1.71 to 1.73. Garnet and Spinel being
isometric, whoever who has basic testing tools (polariscope,
refractometer etc) trends to conclude that any blue singly
refractive stone within this range is Spinel. Because blue spinel is
common and no known blue garnet.
Is there any other technique to identify without x-raying.