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.