The dichroscope uses Polaroid filters to determine if a stone is doubly refractive.
It is a convenient alternative to a calcite dichroscope.
The gemstone should be placed slightly touching the aperture. A strong source of white light such as a pentorch or fiber optic light should be placed behind the gemstone. The pleochroic colors of the gemstone are seperated by the calcite rhomb. Two small windows will be seen next to each other when one looks through the viewing end of the dichroscope.These two windows will be colored. The colors may be the same or different shades of color or entirely different colors. The dichroscope should be turned slowly between the fingers up to 180° while keeping the gemstone in a fixed position. The colors in the two windows should be observed to see if they remain the same or change.The procedure should be repeated four times from different angles.
After examining the gemstone there are three possibilities:
- i) The colors remain the same in both windows even when rotated in all directions.
The stone is isotropic (Single refractive). Only a few colored stones are single refractive.
- ii) Two colors are observed.
The stone is anisotropic uniaxial.
There are two important exceptions. Peridot may appear uniaxial even though it is a biaxial gemstone. Low type zircon may show as isotropic.
iii) Three colors are observed.
The gemstone is anisotropic biaxial.
When three shades of color are seen the stone is biaxial without exception.
Summary of the use of dichroscopes
A dichroscope can be used to differentiate singly refractive gemstones (eg alamandine garnet & blue spinel) from double refractive gemstones with similar colour (such as ruby and blue sapphire respectively).
The double refractive gemstones show the dichroic effect, that is two shades of color will be seen in ruby and blue sapphire but not in garnet or blue spinel.