The dichroscope is a small handheld instrument enclosing a calcite prism. It is used by shining a narrow beam of light through the stone and observing the light coming out of the stone into the dichroscope.
This device enables to separate light rays travelling in different directions. Effectively, this shows the pleochroic colours, when present, which make up the colour of light transmitted in one direction through the crystal. Singly refractive (SR) minerals, within which light travels as a single ray, will only ever display one colour through the dichroscope, regardless of the direction light passes through the crystal; whereas doubly refractive (DR) minerals, which split light in two different rays, will display two different colours through the dichroscope. When those two colours remain the same no matter what direction light passes through the crystal, it implies that the stone is DR uniaxial, that is, it has one optic axis. When a third colour is seen with one of the other previous two in a different crystal direction (usually best observed at 90° to the first direction), it reveals that the stone is biaxial, which means it has two optic axes.
Some gemstones have very similar properties, yet can sometimes be differentiated on the basis of their optic character.
