Overview of the crystal systems and their optical properties

Structure Structure type Crystal axes Angles Symmetry (of highest crystal class) Optic character Refractive index (RI) Optic sign Pleochroism Gem examples Amorphous No order No axes No symmetry Isotropic Singly refractive 1 RI n None None Glass Amber Cubic Isometric: 1 axis length a1 = a2 = a3 All at 90° 13 planes 9 axes Center Isotropic Singly refractive 1 RI n None None Diamond Spinel Garnet Tetragonal Dimetric: 2 axis lengths a1 = a2 ≠ c All at 90° 5 planes 5 axes Center Anisotropic Doubly refractive Uniaxial 2 RIs nω and nε + = nε > nω – = nε < nω May be dichroic Zircon Hexagonal Dimetric: 2 axis lengths a1 = a2 = a3 ≠ c a axes at 60°; c axis at 90° to their plane 7 planes 7 axes Center Anisotropic Doubly refractive Uniaxial 2 RIs nω and nε + = nε > nω – = nε < nω     May be dichroic Beryl Apatite Trigonal Dimetric: 2 axis lengths a1 = a2 = a3 ≠ c a axes at 60°; c axis at 90° to their plane 3 planes 4 axes Center Anisotropic Doubly refractive Uniaxial 2 RIs nω and nε + = nε > nω – = nε < nω     May be dichroic Corundum Quartz Tourmaline Orthorhombic Trimetric: 3 axis lengths a ≠ b ≠ c All at 90° 3 planes 3 axes Center Anisotropic Doubly refractive Biaxial 3 RIs nα, nβ, nγ + = nβ closer to nα – = nβ closer to nγ ± = nβ midway to nα & nγ May be trichroic Topaz Zoisite Olivine (peridot) Monoclinic Trimetric: 3 axis lengths a ≠ b ≠ c 2 axes at 90°; 1 oblique 1 axis 1 plane Center Anisotropic Doubly refractive Biaxial 3 RIs nα, nβ, nγ + = nβ closer to nα – = nβ closer to nγ ± = nβ midway to nα & nγ May be trichroic Orthoclase Spodumene Triclinic Trimetric: 3 axis lengths a ≠ b ≠ c all axes oblique No planes No axes Center Anisotropic Doubly refractive Biaxial 3 RIs nα, nβ, nγ + = nβ closer to nα – = nβ closer to nγ ± = nβ midway to nα & nγ May be trichroic Axinite Labradorite

Optic character/sign with the Refractometer

Optic character/curve variations: Uniaxial or biaxial

1. Two constant curves = Uniaxial
2. Two variable curves = Biaxial
3. One constant/one variable, which meet but do not cross = Uniaxial
4. One constant/one variable which don't meet
5. Check the polaroid angle of the constant curve
   • Biaxial = polaroid angle of constant curve = 90°
   • Uniaxial = polaroid angle of constant curve ≠ 90°

RI readings for different faces on corundum. Corundum is uniaxial negative. Illustration © Richard W. Hughes

Optic sign

Uniaxial stones

1. High RI curve varies = (+)
2. Low RI curve varies = (-)
3. Both curves constant: At 0° polaroid angle, only the o-ray is seen
   • a. If low curve is seen = (+)
   • b. If high curve is seen = (-)

Biaxial stones

1. If n_β is closer to n_α, the gem is (+)
2. If n_β is closer to n_γ, the gem is (-)
3. If n_β is halfway between n_α and n_γ, the gem is (±)
4. If two possible betas exist, false beta will have a polaroid angle equal to 90°. True beta will have a polaroid angle unequal to 90°.

Polaroid angle

• 0° polaroid angle is when the transmission direction of the polaroid plate is parallel to the refractometer scale divisions.
• 90° polaroid angle is when the transmission direction of the polaroid plate is perpendicular to the refractometer scale divisions.

Symbols

Uniaxial crystals

• n_ω = omega, the constant RI of a uniaxial crystal
• n_ε = epsilon, the variable RI of a uniaxial crystal

Biaxial crystals

• n_α = alpha, the lowest RI of a biaxial crystal
• n_β = beta, the intermediate RI of a biaxial crystal
• n_γ = gamma, the highest RI of a biaxial crystal

This page is http://www.ruby-sapphire.com/crystal_optics.htm
Page updated 14 April, 2008

 

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