Formula: α-AlO(OH)
Reflected/transmitted colour: colourless
Pleochroism: strongly coloured specimens (due to iron or manganese substitution) may have absorption α < β < γ
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: α = 1.682-1.706, β = 1.705-1.725, γ = 1.730-1.752
Birefringence δ: 0.04-0.05
2Vγ angle: 84-86°
Dispersion: r < v, weak
Optical orientation: α = z, β = y, γ = x
Optical axial plane: (010)
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