Handheld spectroscope allows you to read a gemstone's spectral signature.
What is detects: absorption spectra.
How to use: view the colour of a gemstone, either by shining a light (pen-torch) through the stone into the spectroscope if the stone is transparent, or by viewing its reflected light if the stone is opaque.
What you see: a spectrum (rainbow) which might be interrupted with black or white lines, it is the pattern of these lines you must learn to recognize. For the gemmologist: best at detecting iron and chromium.
For the non-gemmolgist: the 'diamond simulants' YAG and Cubic Zirconia give spectacular spectra and so too does some Zircon. Other gemstones are more difficult to detect.