It is sometimes possible to estimate the identity of a stone quite accurately by visual observation only. This is explained partly by the peculiar shades of colour recognisable in certain gem species. These can be so unique that they may be distinguished with ease amongst others.
However, there are many situations when colours will appear so much alike others that it is impossible to distinguish one gem from another. Let us consider a brownish-orange spessartine garnet against an almandine garnet of the same colour. If the colours are similar enough, it is virtually impossible to tell them apart. Naturally, other gemmological tests will reveal almost identical RI and SG, as well the same singly refractive optical nature. Although the human eye cannot detect subtle differences between the colours of those garnet species, it does not mean none exists. This is where the spectroscope comes in as valuable assistance. This device enables the observer to view the entire visible spectrum for those colours, i.e. the splitting of the visible light into the spectral colours of the rainbow. The visible spectra of the two garnets would show similarities since the stones are both garnets of the same colour, yet there will also be marked differences related to the nature of each species. In this case, spessartine garnet will display absorption lines (lines of darkness) at the frequencies of 412, 424 and 432 nm, located in the violet part of the visible spectrum. These absorption lines, related to the presence of manganese, will not be present in the visible spectrum of almandine garnet, which does not contain manganese.
