Color centers in amethyst are produced by action of ionizing radiation on precursor centers which arise from substitution of Fe3+ for Si4+ in the α‐quartz structure, with charge neutralization usually achieved by an alkali‐metal ion on the twofold axis of the center. These Fe3+ centers provide the dominant features of the electron paramagnetic resonance spectrum of amethyst, which was measured in the X‐band at room and liquid‐nitrogen temperatures. The spectrum was fitted to an orthorhombic spin Hamiltonian with g=2.0032, D=−2.0463, E=5.5277, F=−0.0282 Gc/sec. The optical biaxiality of amethyst, with biaxial angle 2V=6 to 12°, is due to unequal occupation of the three equivalent Si4+ sites in the structure by Fe3+ during crystal growth. This phenomenon may help to specify geological conditions during hydrothermal growth of natural amethyst.

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