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.
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
Blue Definite Dark blue / greenish blue
Yeellow Weak Yellow / light yellow
Green Weak Green – yellow / greenish – yellow
Pink Definite Light red / yellowish – red
Red Definite Dark red / light red
Green Strong Dark green / yellow green
Blue Strong Dark blue / light blue