Polarisation
Polarisation
The wave-vibration restriction that lets gemmology distinguish isotropic from anisotropic stones
Polarisation is the restriction of light-wave vibrations to a single plane, a fundamental optical phenomenon and the basis of the polariscope, the dichroscope, and the conoscope — three of the standard gemmological instruments. Ordinary light vibrates in all planes perpendicular to its direction of travel; polarised light vibrates in one plane only. The behaviour of polarised light passing through a gemstone reveals whether the stone is optically isotropic or anisotropic, and the determination is one of the foundational diagnostic tests in identification.
The physics
Light is an electromagnetic wave with an electric field vibrating perpendicular to the direction of propagation. In ordinary unpolarised light, the electric-field vibrations are randomly oriented across all azimuthal directions; in polarised light, the vibrations are restricted to a single plane. A polarising filter — a sheet of polymer with embedded oriented crystals or molecules — transmits the component of incident light vibrating parallel to its transmission axis and absorbs the component vibrating perpendicular to it. Light emerging from a polariser is therefore plane-polarised in the filter's transmission direction.
When two polarising filters are placed with their transmission axes at right angles (crossed polars), the second filter blocks all light from the first, since the incident polarised light has no component parallel to the second filter's axis. The crossed-polar configuration is dark unless something between the filters disturbs the polarisation state of the transiting light.
Anisotropic stones in crossed polars
An anisotropic gemstone — quartz, ruby, sapphire, tourmaline, topaz, and most other faceted stones — placed between crossed polars splits incident polarised light into two orthogonally polarised rays travelling at different velocities (double refraction). On exiting the stone, the two rays recombine, and the recombined light has a polarisation state that generally differs from the original incident polarisation. The second filter transmits whatever component of this altered polarisation aligns with its axis, and the stone appears bright. Rotating the stone 90 degrees brings the optical axes into alignment with the polariser axes, the splitting vanishes, and the stone goes dark. Through 360 degrees of rotation, an anisotropic stone alternates between bright and dark four times.
An isotropic stone — garnet, diamond, fluorite, glass, spinel — does not split polarised light. The polarisation state of light passing through the stone is unchanged, and the second filter blocks the light just as it did with no stone present. The stone remains dark throughout 360 degrees of rotation.
The dichroscope and pleochroism
A second polarisation-based instrument, the dichroscope, uses a single calcite rhomb to separate light passing through a stone into two polarised rays presented side by side. The two rays show the stone's behaviour along two orthogonal directions, and any difference in colour between them — pleochroism — appears immediately as different colours in the two halves of the dichroscope window. Pleochroism is restricted to anisotropic stones and is highly diagnostic: tanzanite, tourmaline, alexandrite, and ruby all show distinctive pleochroism that the dichroscope reveals.
In the trade
Polarisation-based testing is the fastest and most informative single class of gemmological observation. The polariscope determines optic character in seconds and excludes major species classes; the dichroscope adds species-specific colour information. For Skyjems and other working dealers, the routine application of these tests is part of the daily discipline of identifying material and detecting simulants — a routine the trade depends on and which polarisation theory makes possible.