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When light passes through a doubly refractive gemstone, the light is split into two rays which are polarized at right angles to each other and travel at differing velocities through the gemstone. In some coloured doubly refractive gemstones, these rays may emerge differing in shade or colour. When this occurs, the rays are said to have experienced "Differential Selective Absorption".
Differential Selective Absorption:
Variations in the absorption of certain wavelengths dependent on direction, causing a stone to appear differently coloured in different directions.
D.S.A in :
Singly refractive Gemstones: No D.S.A, stone is the same colour in all directions. Monochroic.
Doubly refractive Gemstones:
Uniaxial - Two colours seen. Dichroic.
Biaxial - Three colours seen. Trichroic
Another term used for Differential Selective Absorption is Pleochroism.
An instrument comprising of a suitably cut rhomb of Iceland Spar (Calcite) and a lens system in a short tube with a small square aperture at the other end.
The dichroscope separates the polarized rays so that they may be observed side by side.
Dichroism can also be used in colour grading coloured gemstones.
i.e Blue Sapphire described as Greenish/BLUE
Dichroism can also be detected by using a polarizing filter. If it is rotated against a gem, first one colour will be seen (caused by one polarized ray) and then another. It can also be seen simultaneously by using a piece of suitably cut polaroid.