The colour of ruby is due to a trace of chromic oxide (Cr2O3), which enters the crystal structure by small-scale isomorphous replacement of some of the alumimium atoms. The amount, about 1–3%, determines the depth of colour, but the presence of ferric iron also modifies the tint, reducing fluorescence and giving the rubies from Thailand a characteristic brownish tinge.
The finest rubies will be a strongly fluorescent red, resembling the colour of a red traffic signal. Such stones often contain extremely fine particles which serve to scatter light onto all facets and reduce extinction (facets which are dark). In the past in Myanmar this colour was termed ‘pigeon’s blood’, but the term has little meaning today as so few people have seen the blood of this Burmese bird.