  The main purpose of this research is dedicated to the ESR study of the
impurity ions such as Gd3+ in the natural zircon crystal because the presence
of the impurity ions in the crystal usually causes color in crystal. The Zeeman
interaction, the weak hyperfine interaction and the crystal field interaction
due to the environments of the impurity ions will be calculated. Furthermore,
the relation between the resonance magnetic field positions in the ESR spectra
and the applied magnetic field directions will reflect the impurity ion site
symmetry. The change in the crystal field parameters of heat-treated zircon at
different atmospheres were related to the change in the color of zircon.
