Spectroscope (diffraction grating type), 50mm long X 12mm dia.
You view the light from coloured stones, you see a spectrum of colour (rainbow) interrupted with black (or white) lines*. You don't take any measurements (this is not a spectrometer), you just have to recognise the patterns of lines (see the example in the picture).
If you don't have any gemstones to hand, here's some fun things to do with a spectroscope. Point it at the sky and see the lines that correspond to hydrogen burning in the sun and oxygen being absorbed by the atmosphere. Point it at a fluorescent light and see the dozens of lines caused by the gases burning inside the tube. Shine a torch through your finger, see the pattern of lines caused by iron in your blood.
* This is because some elements (transitional elements) absorb specific wavelengths (colours) of light, appearing as black lines; some re- transmit specific wavelengths, appearing as white lines. These patterns of lines give clues as the chemical composition of the stone.
Is it easy to use?
No. It can be fiddly, holding the stone, a light (e.g. small torch) and the spectroscope, all at the correct angle to view the colour.
If you like colours and are good at recognising patterns of lines, you'll love this tester. Get to recognise the patterns of lines that indicates chromium and iron, instantly identify blue manmade stones (cobalt-blue), see the myriad of lines in some zircon caused by radioactive decay. But if you have no sense of colour whatsoever (like me!) it will be hard work.
It works by looking at the colour of a stone, so the stone must have colour, the stronger the colour the clearer the spectrum.
Usually you would shine a light through a transparent stone, but you can examine opaque stones by 'bouncing' the light off the surface of the stone into the spectroscope, but this is difficult, especially if the stone doesn't have a strong colour.
This is a standard gem tester used by gemmologists, it does not come with instructions, you are expected to know how to use it. However, if you want to know what you will see click here for a link to a website with hundreds of examples you can download.