One of the main attractions of a mineral specimen is its colour and this is often due to the chemical composition of the mineral. Commonly, transition metals (nickel, chromium, copper and so on) give rise to colours that are attractive to the eye. The mineral in question absorbs certain wavelengths of the visible light spectrum and emits only those that you can see. For instance, the unmistakable blue of dioptase (a hydrous silicate of copper) is due to the presence of copper. Malachite (a mixed copper hydroxide/carbonate) has a characteristic green that is also due to copper. The atomic structure of this mineral, which is totally different from dioptase, illustrates how colour does not always indicate composition.
But what gives rise to the colour of opals? These hydrous silicas contain virtually no trace elements yet display colours ranging from the reds of the Mexican Fire opals through to the blues of the Australian opals. So, why should a substance of such simple composition display such attractive hues? The work of (amongst others) Jones Sanders (1964) and Segnit (1971), who used an electron microscope to look at very thin slices of opal, show that these minerals are composed of spheres of hydrated silica. The colour is a result of daylight being diffracted by the spheres and the wavelength observed depends on their size. Where there are similar size spheres, the colour is uniform and but mixed colours are generated by mixed sized spheres.
Jones J.B. Sanders J.V. and Segnit E.R. (1964). Structure of opals
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Jones J.B. and Segnit E.R. (1971) The nature of opal. 1. Nomenclature and constituent phases. Journal of the Geological Society of America, 18, 57 -68.