Refractive index (RI) measures how strongly a material bends light as it passes from air into the stone. The higher the RI, the more the light bends, and the more the stone interacts with light in ways that produce brilliance, fire and optical character. Diamond’s RI of 2.417 is the highest of any natural transparent gemstone; fluorite’s RI of 1.434 is among the lowest. The spread across gemstones covers a range that, when combined with birefringence measurement, identifies or eliminates the majority of species without any further testing.
Reading the refractometer
The standard gemological refractometer provides readings between approximately 1.40 and 1.80, the range visible through the eyepiece shadow boundary. The shadow boundary appears as a sharp dark/light division on the scale; the number where this falls is the RI. For singly refractive stones (cubic crystals and glass), one reading appears regardless of how the stone is rotated. For doubly refractive stones (all non-cubic crystals), two shadow boundaries appear as the stone is rotated, a lower and a higher reading. The difference between the two is the birefringence.
Birefringence as a second identifier
Birefringence is the difference between a stone’s maximum and minimum refractive indices. It is characteristic of the species and adds substantial information to the RI reading. Zircon, for example, has an RI range of 1.925ÔÇô1.984, over the standard refractometer limit, but its birefringence of 0.059 is distinctive when it can be measured. Calcite’s birefringence of 0.172 is so high that doubling is visible to the naked eye through the stone; tourmaline’s birefringence of 0.018 is lower but still clearly detectable. Combining the RI reading with birefringence eliminates most confusion between stones in the same RI range.
Practical limitations
The refractometer requires a flat, polished surface. Rough stones cannot be read directly. Curved surfaces produce approximate readings. Stones with RI above approximately 1.80 fall over the refractometer’s limit and read at the edge of the scale. Zircon, diamond, cubic zirconia and some garnets all read over the limit, which itself is diagnostic. Reading accuracy depends on calibration with a standard reference liquid (typically methylene iodide at RI 1.74) and clear shadow boundaries. Some inclusions or surface damage can blur the reading.
What a single RI reading tells you
A reading of 1.54 with minimal birefringence places the stone in a range shared by quartz, glass and fluorite. A reading of 1.76ÔÇô1.77 with birefringence around 0.008 places the stone firmly in corundum, sapphire or ruby. A reading of 1.690ÔÇô1.702 with birefringence of 0.009 places the stone in tanzanite. A reading over the limit with high birefringence visible points to zircon. The refractometer does not identify stones alone. It eliminates groups and narrows the field for subsequent testing with dichroscope, spectroscope, density measurement and microscopy. Used in sequence with other instruments, it is the most efficient starting point in any identification workflow.