A gemstone or mineral that displays a different color depending on the orientation of the stone is said to be pleochroic. This phenomena is the result of different wavelengths of light being absorbed in different amounts in different directions. If the light traveling through a stone stays in one ray, that stone is singling refractive. If light divides into two rays, it is said to be double refractive (or dichroic.) In a few rare cases, stones will show 3 colors. These are called trichroic.
Dichroscopes are used to determine pleochroism in gemstones. It can also be used on rough (or uncut stones) to id or determine the best orientation for cutting the stone.
- The strength of the colors shown is also important to identification. Weak or strong pleochroism play a role in helping determine the identity of the gemstone.
- Take care in choosing your light source, flourescent light can give a false reading for very weak pleochroism even in stones that do not have pleochroism.
- The aperature end of the dichroscope looks like a hex nut. To know which end you look through, just remember there should be a nut at each end!
- Remember: Always complete at least three tests when identifying a stone. No one test is conclusive.