Refractive Index Calculator

A refractometer reads the critical angle at which light passing from the gemstone into a high-RI contact liquid transitions from refracting to totally internally reflecting, and displays that angle's corresponding RI value directly on a scale as a shadow edge. For an anisotropic stone, rotating it on the hemicylinder makes the shadow edge split into two positions as light polarized along different crystal directions bends by different amounts, and the distance between those two positions is what this calculator records as birefringence.

Zero or near-zero birefringence (below about 0.001 in this calculator) means the stone is optically isotropic -- light travels through it at the same speed regardless of direction, which is characteristic of cubic-crystal-system materials like diamond, garnet, spinel, and cubic zirconia, along with amorphous materials like glass. A single, non-splitting shadow edge on the refractometer is the practical signature of this, and it immediately rules out the many gem species that are doubly refractive.

Peridot's birefringence runs around 0.036 (GIA lists a published range of 0.035 to 0.038), roughly three to four times higher than topaz's typical 0.008 to 0.010, because its crystal structure separates the two refracted rays by a much larger margin. That difference is visible to the naked eye through a loupe as clearly doubled back-facet edges in peridot, a diagnostic trick working gemologists use before ever touching a refractometer.

Yes, and that overlap is exactly why this calculator returns a best match plus a list of other close matches rather than a single verdict. Species with adjoining or overlapping RI windows -- such as certain glass compositions sitting inside the amethyst/quartz range, or lower emerald readings brushing against other beryllium silicates -- cannot be separated by RI and birefringence alone, and need additional tests like specific gravity or spectroscopy to confirm.