The external Refractometer is one of the most essential gemological instruments, used to measure the refractive index (RI) and estimate the birefringence of gemstones. By applying contact liquid and using the critical angle method, it helps gemologists identify gemstones, verify authenticity, and distinguish between natural, synthetic, and treated stones. This tool is especially effective for transparent and semi-transparent gems and can also be applied using the spot method on mounted stones.
