Prism spectroscope

Specifications

Prism Spectroscope
-High resolution
-Black appearance

-Adjustable focus
-Aviod reflecting interference
-Leather package

Absorption

Color as perceived by the human eye consists of the 7 colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo and Violet. All these colors travel at different speeds and have their own wavelengths. When all the above colors combine, we see it as white light.

When white light reaches a substance, part of the light components may be absorbed by the substance. The other light components (residue) form the color of that substance. For instance, if a gemstone absorbs all the colors of the rainbow except red, only the red part of the original white light will be visible, and the gemstone will therefore be red.
When viewed through a spectroscope, the absorbed parts of light by that gemstone will disappear from the spectrum image and only red will be visible in the prism of the spectroscope.
Likewise if all colors except red and blue are absorbed by a gemstone, the residual colors (red and blue) will give rise to a purple gemstone.

The pictures below give a crude example of both above-mentioned situations.

Prism spectroscope

The prism spectroscope is based on dispersion. The light enters through a narrow slit (some models allow you to adjust the width of the slit) and is then dispersed through a series of prisms. Some models have an attachment with a built-in scale. These models are generally more expensive than their diffraction type cousins.
Because prism spectroscopes are based on dispersion, the blue area of the spectrum is more spread out and the red parts are more condensed than the diffraction grating types.

Use of the spectroscope

Using the spectroscope poses many problems for those who are not familiar with the instrument. Therefore, before attempting to determine the absorption spectra of gems, it is best to hold the spectroscope against some different sources of illumination, such as a fluorescent light bulb, a computer monitor, etc. This will show you very clear absorption bands in most cases.

Proper use of the spectroscope and lighting is vital when wanting to see good spectra of gemstones.
The most widely used technique is to make use of reflected light. Light enters the pavilion of a gemstone at a 45 degree angle and the spectroscope should be placed at the same angle on the other side.
The light will travel its longest possible path in this way, picking up the most color.

To prevent the background on which the stone lays from causing false readings, one should use a black non-reflective underground, such as a small piece of black velvet.

Another technique is to position the gemstone and the light source (penlight) in one hand in such a way that the light source illuminates the gem from behind, thus viewing the gemstone in transmitted light.

There are nice spectroscope stands (some with built-in illumination) on the market, but gaining some experience eliminates the need for them.
For the new user, it is recommended to start with a gemstone that produces a clear absorption spectrum, such as synthetic ruby.