Fit #1— Cauchy from (n_D, V_D estimated from phosphate family)
Cauchy approximation
Valid 0.38–0.78 µm
n
n = 1.6800VD = 60.0
n(λ)=1.6629+0.005933/λ2
Fe-Al phosphate hydrate. Dispersion estimated. n_D from published gemological tables; V_D estimated from phosphate family because species-specific dispersion is not widely reported. Source: Winchell / GIA / Liddicoat compilations.