Optical Properties

Refractive index, birefringence, pleochroism, dispersion, and other optical properties of gemstones.

By Fabian Moor Last updated
refractometer identification optics pleochroism

Introduction

Optical properties are the primary diagnostic features used in gemmological
identification: refractive index (RI), birefringence, optic character, pleochroism,
dispersion, and absorption spectrum together form the fingerprint of each gem
species. Birefringence (the difference between the maximum and minimum RI of
an anisotropic gemstone) is calculated as BR = RI_max − RI_min. All gems except
those in the cubic system and amorphous materials (glass, opal) are anisotropic
and therefore birefringent. High birefringence (above 0.020) is visible as
doubling of back facets when viewed through a 10× loupe: zircon (0.059) and
calcite (0.172) show dramatic doubling, while quartz (0.009) and beryl (0.006)
show minimal doubling. A standard gemmological refractometer measures both RI and
birefringence directly by rotating the stone 90° and reading both shadow-edge
positions on a scale of 1.35–1.81. A measured value outside the published range
for a suspected species immediately excludes that identification, making optical
properties the first line of testing. [1]

Refractive Index (RI)

The refractive index measures how much light slows down when entering a gemstone.
It's calculated as the ratio of light speed in vacuum to speed in the material.

A standard gemmological refractometer can measure RI values from approximately
1.35 to 1.81 (limited by the contact liquid). Gems with higher RI require
alternative methods. Reference RI ranges for each species are documented in
standard gemmological tables. [2]

Optical Properties of Common Gem Materials
Material RI Birefringence Optical Character
Amber 1.54 approx. - I
Andalusite 1.63-1.64 0.007-0.013 B-
Apatite 1.63-1.64 0.002-0.006 U-
Beryl varieties 1.56-1.60 0.003-0.010 U-
Calcite varieties 1.48-1.66 0.172 U-
Chrysoberyl 1.74-1.76 0.008-0.010 B+
Corundum varieties 1.76-1.78 0.008-0.009 U-
Cubic zirconia 2.17 approx. - I
Diamond 2.42 - I
Diopside 1.67-1.70 0.024-0.030 B+
Feldspar varieties 1.52-1.57 0.004-0.009 B+/-
Fluorite 1.43-1.44 - I
Garnet, almandine 1.76-1.81 - I
Garnet, demantoid 1.89 approx. - I
Garnet, grossular 1.73-1.75 - I
Garnet, hydrogrossular 1.70-1.73 - I
Garnet, pyrope 1.74-1.76 - I
Garnet, spessartine 1.79-1.82 - I
Iolite 1.54-1.56 0.008-0.012 B-
Natural glass 1.50 approx. - I
Opal 1.40-1.46 - I
Paste (artificial glass) 1.50-1.70 - I
Peridot 1.65-1.69 0.036 B+/-
Quartz, crystalline 1.54-1.56 0.009 U+
Quartz, polycrystalline 1.53-1.55 - -
Rhodochrosite 1.59-1.82 0.220 U-
Scapolite 1.54-1.58 0.009-0.026 U-
Sinhalite 1.67-1.71 0.037-0.038 B-
Sphene 1.88-2.05 0.105-0.135 B+
Spinel 1.71-1.74 - I
Spinel, Verneuil synthetic 1.72-1.73 - I
Spodumene 1.66-1.68 0.015-0.016 B+
Synthetic moissanite 2.65-2.69 0.043 U
Tanzanite 1.69-1.70 0.006-0.013 B+
Topaz 1.61-1.64 0.008-0.010 B+
Tourmaline 1.62-1.65 0.014-0.021 U-
YAG 1.83 approx. - I
Zircon 1.78-1.99 up to 0.059 U+

Birefringence

Birefringence is the difference between maximum and minimum RI values in
anisotropic (non-cubic) crystals. Light entering the stone splits into two
rays travelling at different speeds.

Formula: Birefringence = RI(max) - RI(min)

Example (Quartz): 1.553 - 1.544 = 0.009

Diagnostic Birefringence Values
Birefringence Gemstones
None (0.000) Diamond, Spinel, Garnet (isotropic)
Low (0.004-0.010) Quartz (0.009), Beryl (0.006)
Medium (0.010-0.020) Topaz (0.010), Tourmaline (0.018)
High (0.020-0.050) Peridot (0.036), Zircon (0.059)
Very High (>0.050) Calcite (0.172), Sphene (0.134)

Doubling Effect

Optic Character and Sign

The polariscope and conoscope reveal a gem's optic character.

Character Crystal Systems RI Values
Isotropic Cubic Single RI (n)
Uniaxial Hexagonal, Trigonal, Tetragonal Two values (ω, ε)
Biaxial Orthorhombic, Monoclinic, Triclinic Three values (α, β, γ)

Optic Sign

  • Uniaxial +: ε > ω (quartz, zircon)
  • Uniaxial -: ε < ω (corundum, tourmaline, beryl)
  • Biaxial +: β closer to α (topaz)
  • Biaxial -: β closer to γ (peridot)

Pleochroism

Pleochroism is the absorption of different colours in different crystal directions.
Viewed with a dichroscope, anisotropic coloured gems show different colours.

Gemstone Pleochroism Colours
Ruby Strong Purple-red / Orange-red
Sapphire (blue) Strong Violet-blue / Green-blue
Tourmaline Strong Variable (often dark/light)
Tanzanite Strong (trichroic) Blue / Purple / Bronze
Peridot Weak Yellow-green / Green
Aquamarine Weak Near colourless / Blue

Dispersion

Dispersion is the separation of white light into spectral colours ("fire").
It's measured as the difference in RI between red (B) and violet (G) wavelengths.

Gemstone Dispersion (B-G) Fire Quality
Diamond 0.044 Strong
Sphene 0.051 Very strong
Demantoid garnet 0.057 Very strong
Zircon 0.039 Strong
Spinel 0.020 Moderate
Quartz 0.013 Low

Absorption Spectrum

The absorption spectrum shows which wavelengths of light are absorbed by a gem.
Viewed through a spectroscope, absorption bands appear as dark lines or bands.

Diagnostic Spectra:

  • Ruby: Lines at 694nm (doublet), 668nm, 659nm [3]
  • Emerald: Line at 683nm (Cr), bands in red/yellow
  • Blue sapphire: Bands at 450nm, 460nm, 470nm [4]
  • Almandine garnet: Bands at 505nm, 520nm, 575nm [1]
  • Zircon: Fine lines throughout spectrum (uranium)

Practice with Interactive Tools

References

  1. ↑ 1. Read, P. (2014). Gemmology (3rd ed.). Routledge. DOI: 10.4324/9780080507224.
  2. ↑ 2. Schumann, W. (2013). Gemstones of the World (5th ed.). Sterling Publishing. ISBN: 978-1-4027-6829-3.
  3. ↑ 3. Nassau, K. (2001). The Physics and Chemistry of Color: The Fifteen Mechanisms (2nd ed.). Wiley-Interscience. ISBN: 978-0-471-39106-7.
  4. ↑ 4. Dubinsky, E. (2020). A Quantitative Description of the Causes of Color in Corundum. Gems & Gemology, 56(1). DOI: 10.5741/gems.56.1.2.

Related Topics

Crystal SystemsPhysical PropertiesAdvanced Optical Properties