Watermelon tourmaline (Fluor-elbaite)

General

Category: Cyclosilicate

Chemical formula: Na (Li1.5,Al1.5) Al6,Si6,O18 (BO3)3(OH)3F


Identification

Mohs scale hardness: 7–7.5

Specific gravity: 3.06 (+.20 -.06)

Optical properties: Double refractive, uniaxial negative

Refractive index: nω=1.635–1.675, nε=1.610–1.650

Birefringence: -0.018 to -0.040; typically about .020 but in dark stones it may reach .040

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