Spectroscope

by Ritika Changrani

A Spectroscope helps to identify cut stones, rough stones, mounted stones and unmounted stones to see whether they are real or fakes.

A Spectroscope is a gemological device used to test if the gemstone is natural or synthetic. Spectroscope helps to determine what parts of white light are being absorbed by the gemstone.

White light is made up of seven different colours namely violet, indigo, blue, green, yellow, orange and red, which have different wavelengths and travel with different speed. When white light is passed through a gemstone, a part of it, based on the energy level is absorbed by the gemstone and the remaining colours combine to give a different colour to the stone. When observed through the spectroscope, one can see bands or colours missing. The missing bands are the colours absorbed by the gemstone. The colours absorbed let us know the elements present and the chemical composition in the gemstone which thereby helps in gemstone identification.

Types of Spectroscope

There are two types of spectroscope, diffraction grating spectroscope and prism spectroscope.

Diffraction Grating Spectroscope

Diffraction grating spectroscope implies the principle of diffraction. Diffraction refers to the bending of light waves around a sharp edge or an obstacle by transmission or by reflection. This type of spectroscope is made up of high-dispersion diffraction grating film, movable slits and a photo detector which is used to measure the properties of light within the specified portion of the spectrum. Here light is made to enter a narrow slit. The light is then diffracted by thin high-dispersion diffraction grating film. This causes uniform spectrum image and disperses light into large visible spectrum. The advantage of diffraction grating spectroscope over the prism spectroscopes is that the spectrum is evenly distributed and hence is easier to read. But the spectrum thus produced is not as bright as that produced by prism spectroscope. Diffraction grating spectroscopes do not have an inbuilt calibration scale.

Prism Spectroscope

Prism spectroscope works on the principle of dispersion. Dispersion is defined as the splitting of white light into its constituent colours. Prism spectroscope is made of three optical grade glass prisms placed in optical contact with each other. Most of the prism spectroscopes come with calibration scale. But experienced people make observations without the scale.

In a prism spectroscope, light is made to enter through a narrow slit which is dispersed by passing through a series of prisms. In prism spectroscopes, the spectrum obtained is brighter and faint lines are clearly visible. But the disadvantage with prism spectroscope is that the spectrum is not evenly distributed. Blue parts are more spread and red parts are more condensed. It is difficult to distinguish lines in red part since they are very close to each other. Prism spectroscopes come with focus slide control and light slit control which allows adjusting for the amount of light entering the unit.

How to use a Spectroscope?

• First of all, before testing for the absorption spectra of the gemstones, hold the spectroscope against some different sources of illumination. Holding it against either a fluorescent light bulb or a computer monitor shows clear absorption bands.
• Place the gemstone on a black non-reflective surface may be a piece of black velvety cloth. If the surface is non-reflective, the observations may not be accurate and hence give false readings.
• Place the source of white light such that light enters the pavilion of gemstone at an angle of 45 degrees.
• Place the spectroscope on the other side at the same angle. The angle of 45 degree is chosen since this is the path in which light travels in the longest path picking up most colours.
• The other way of positioning can be by placing the light source and the gemstone in one line such that the light illuminates the gemstone from the back. This helps to view the gemstone in transmitted light.
• Note the observations through the spectroscope. Note down the spectrum seen with red end of the spectrum on the left and violet end of the spectrum to the right. The colours which are absorbed by the gemstone appear as missing bands. The colours absorbed mix up to give the gemstone a different colour.
• Based on the spectrum obtained, you can analyze the composition of the gemstone and hence determine the type of gemstone.
• The spectrum can also be compared with that of a known gemstone to identify if the gemstone is a natural or a synthetic one.

Though spectroscope is one of the important tools in gemology, it should be purchased only when it is extremely necessary. The cost of spectroscope is quite high. Furthermore, only experienced gemmologists can make use of the spectroscope more efficiently. It is difficult for an inexperienced person to study the nature of gemstones using a spectroscope.