Alluvial kornerupine testing: optical absorption lines and extreme trichroism in Kolonne?

Kornerupine is an uncommon magnesium-aluminum borosilicate found in metamorphic gravels around Kolonne and Embilipitiya. Faceted stones frequently resemble chrome tourmaline or enstatite. Gemmologists verify kornerupine by measuring its diagnostic optical constants: it is biaxial negative with a refractive index of alpha = 1.667 and gamma = 1.680, yielding a birefringence of 0.013. Under a calcite dichroscope, it displays vivid, undeniable trichroism: emerald green, reddish-brown, and pale yellowish-brown. Chrome diopside is biaxial positive with a higher RI (1.675-1.701), while enstatite displays lower birefringence (0.009) and a 505nm absorption band.