Zultanite (gem-quality color-change diaspore) has a refractive index range of 1.682 to 1.752 with strong birefringence (~0.048). It is a biaxial positive gem with three distinct refractive indices: nα 1.682–1.706, nβ 1.705–1.725, nγ 1.730–1.752. The high birefringence produces visible doubling of pavilion facet edges when viewed through the table.

The three indices

Diaspore is orthorhombic, which means light travels at three different speeds along three principal optical directions. Each direction has its own refractive index:

• nα (alpha): 1.682 to 1.706 — slowest principal direction
• nβ (beta): 1.705 to 1.725 — intermediate
• nγ (gamma): 1.730 to 1.752 — fastest

Birefringence and doubling

The difference between the highest and lowest refractive indices (~0.048) is called birefringence. In Zultanite this is high enough to produce visible doubling of pavilion facet edges when the gem is examined through the table — a diagnostic feature distinguishing it from singly-refractive lookalikes like glass.

Why it matters for the buyer

The high refractive index gives Zultanite strong brilliance — light entering the gem refracts steeply and reflects efficiently off the pavilion facets. The optical signature is also a primary identification tool used by gemological laboratories.