Zultanite is biaxial positive: light traveling through the crystal splits into two rays with three principal refractive indices (nα 1.682–1.706, nβ 1.705–1.725, nγ 1.730–1.752), with nβ closer to nα than to nγ. This is a primary diagnostic feature distinguishing diaspore from singly-refractive simulants and from biaxial-negative competitors. The optic axes are inspected on a polariscope; orientation drives the cutter's table choice.

The measurement

Polariscope examination + interference figure on a conoscope is the standard method documented in laboratory reference texts. Diaspore-specific values are documented across the property guides; this entry focuses on the practical implications.

Implications for buyers and graders

The value above is part of the verified mineralogical baseline editors apply when grading or authenticating a stone. Discrepancies from this baseline are documented case-by-case in laboratory reports and should be flagged before transaction.

Where this fits in the larger archive

See the related property guides for adjacent measurements and the cluster-support articles for practical applications.