FIFTY SHADES OF GREY TOURMALINE – PLEOCHROISM

Perhaps fifty shades are being a little ethusiastic, however, there is a more valid reason why a variety of gemstones come in vastly different shades of their true body colour.

Some gemstones appear to have different colours or depth of colour when viewed in different directions.

This effect is caused by differing absorption of light rays in doubly-refractive crystals.  Where only two main colours are seen, this is termed ‘diochroism’, where three colours are viewed, the term is ‘trichroism’ or ‘peochroism’.  The latter term being a collective description used for both kinds of the multi-colourdness

Pleochroism, derived from Pleo (Greek) meaning many and Chros (Greek) meaning colour, is an optical phenomenon in which some crystals appear to be different colour when observed at different angles, especially within polarized light.

Appearance of pleochroism can be defined as weak, definite, or strong and this is taken into consideration during the cutting process in order to avoid poor colours, or shades that would be too dark or too light.

When crystals are viewed through their vertical axis, they appear darker in colour, when seen through their horizontal axis.  Tourmalines are pleochroic and colour may differ when viewed through different angles of the crystal.

The diagnostic instrument used is the Dichroscope.  It helps t determine behaviours f colours in some gemstones, eg: ruby and sapphire are doubly refractive and will display the dichroic effect.  When looking through the dichroscoe the two differing shades of colour will be displayed.  Ruby and blue sapphire are diochroic were as garnet and blue spinel are non-diochroic and ca be distinguished from each other in this way.

A divided light beam follows different paths within the crystal and travels at different speeds.  The light passing through the crystal will show different colours when viewed from differing angles.  Gemstones are, therefore cut and set either to enhance their diochroic or pleochroic feature, or hide it, depending upon their colours and attractiveness.

 

Gemstones and their pleochroic colours:

Citrine                          Weak               Yellow / pale yellow

Amethyst                      Weak               Purple / grey – purple

Aquamarine                  Strong              Near colourless / light blue / blue – sky blue

Peridot                          Weak               Colourless – pale green / lively green – oily green

Rose Quartz                  Weak               Pink / pale pink

Rutile Quartz                Various             Red – brown / yellow / green

Ruby                            Strong               Yellow – red / deep carmine red

 

Sapphire:

Blue                 Definite             Dark blue / greenish blue

Yeellow             Weak               Yellow / light yellow

Green               Weak               Green – yellow / greenish – yellow

 

Tourmaline:

Pink                  Definite            Light red / yellowish – red

Red                  Definite             Dark red / light red

Green               Strong               Dark green / yellow green

Blue                 Strong               Dark blue / light blue