Natural topaz crystals of gem quality and orange-red colour from the Ouro Preto area, Brazil have been analysed chemically and investigated by optical absorption and electron paramagnetic resonance measurements to elucidate the reason of the special orange-red colour. In all topazes the transition elements V, Mn, Ti, Cr and Fe were found. V occurs as V4+. Mn seems to be substituted as Mn3+ not detectable by EPR at X-band frequencies and Ti as Ti4+ which is inactive for both EPR and optical absorption. Cr and Fe have been found in the valence states Cr3+ and Fe3+, respectively. The orange-red colour is related to two electron centres with geff-values of 1.96 and 1.91, respectively, because both the electron centres and the absorption bands responsible for the orange-red colour can be bleached by thermal treatment whereas the EPR of V4+, Cr3+, Fe3+ is not remarkably influenced by heating the samples up to 600 °C.

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