Light, colour and phenomena
Goal: Understand RI, double refraction, dispersion, pleochroism, colour causes and optical effects.
Refractive index (RI)
How much light slows and bends entering the gem. Diamond 2.417, corundum 1.76–1.77, quartz 1.54–1.55, opal ~1.45.
Single refraction (SR)
One RI. Cubic + amorphous: diamond, spinel, garnet, glass, opal.
Double refraction (DR)
Light splits into two rays with two RIs. Difference = birefringence.
Seeing doubling
Look through the table at the back facets, slightly off-axis. Strong doubling: zircon (0.059), peridot (0.036), sphene, moissanite. None: diamond, spinel, garnet.
Optic character
Uniaxial (U)
Tetragonal, hexagonal, trigonal. One direction (optic axis) with no double refraction. Sign + or −.
Biaxial (B)
Orthorhombic, monoclinic, triclinic. Two optic axes. Sign + or −.
Aggregate (AGG)
Many crystals: stays light in polariscope; spot RI only.
Dispersion (fire)
Pleochroism
DR gems can show different colours in different directions. Dichroic (uniaxial: 2 colours) or trichroic (biaxial: up to 3). Iolite, tanzanite, andalusite and tourmaline show it to the naked eye. Single refractive gems show none.
Why gems are coloured
Idiochromatic
Colour from a main element: peridot (Fe), malachite & turquoise (Cu).
Allochromatic
Trace impurity: Cr in ruby/emerald, Fe+Ti in sapphire, V in tsavorite.
Colour centres
Radiation defects: smoky quartz, blue topaz, Maxixe beryl (can fade).
Physical optics
Diffraction (opal), interference (labradorite), inclusions (aventurescence).
Phenomena
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