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 colored doubly refractive gemstones, these rays may emerge differing in shade or color. 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 colored in different directions.
Singly refractive Gemstones: No D.S.A, stone is the same color 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.
- Place the gemstone on the rotating platform of the dichroscope holder table down.
- Direct light onto the gemstone.
- View the stone through the dichroscope.
- Slowly turn the rotating stage so as to view to all directions.
- Look for different colors or shades of the color.
- Dichroism is only seen in “Doubly Refractive” gemstones.
- It is not seen in gemstones that display “Anomalous Birefringence”.
- May be very weak or even undetectable.
- Not seen in colorless doubly refractive gemstones or along an optic axis.
- Exact colors are not important.
- Helpful in distinguishing:
- Blue Sapphire from Synthetic or Natural Blue Spinel.
- Ruby from Garnet or Spinel.
- Emerald from Demantoid or Tsavorite Garnet.
- Any doubly refractive gemstone (i.e. Amethyst) from Glass.
Dichroism can also be used in color grading colored 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 color 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.