Dichroscope show whether or not a stone is dichroic (reflects 2 colors). This can be helpful in differentiating stones of the same color from one another.
The advantage of the dichroscope is that the two pleochroic colors that may be characteristic of a given direction in a doubly-refractive gem are seen side by side. If no dichroism is identified in the first assessment, the stone can be turned and viewed in other directions. Trichroic gems can also be distinguished and will reveal three colors if the stone is viewed in three directions perpendicular to each other.