Spectroscopes are one of the most difficult gem instruments to master. They come in two varieties: prism and diffraction grating. There is no clear preference between the two, as each has its advantages and disadvantages.
Diffraction grating spectroscopes are the least expensive. Their primary shortcoming is that the display is so dark you cannot get a reading on some stones. Prism spectroscopes have a brighter display and some have a calibrated scale. Their spectrum is spread out unevenly, making it hard to read in the blue end.
Both are difficult to use. Prism spectroscopes have an adjustable light opening and a focusing control. It requires two hands to make these adjustments. To accomplish this, the stone needs to be in a fixed position and most gemologists prefer the instrument attached to a stand as well.
Diffraction grating spectroscopes are usually hand held. Not having to adjust the focus and light opening simplifies the process, but getting the scope, stone, and light source aligned is something less than easy. One needs steady hands and a lot of patience to preform this task gracefully.
The most difficult part of using either kind of spectroscope is getting the instruments aligned with the light…