A dark blue and intense color is reported in other crystals systems, e.g., spinels or silicates such as Co-akermannite Ca2CoSi2O7, when Co 2+ occupies tetrahedral position. Pale green color changes were reported for high purity Co-doped garnets, i.e. Y3Fe5O12, Y3Ga5O12 and Y3Al5O12 with 𝐼𝑎3𝑑 symmetry with mainly Co 3+ and minor amounts of Co 2+ occupying tetrahedral sites. Color changes -in comparison with literature observations -may indicate the presence of Co in tetrahedral coordination. A change from yellow to dark blue in dependence on temperature additionally may indicate a change in valence state due to changing oxygen fugacity with Co 2+ stabilized at higher temperatures (dark blue samples). Further evidence on the influence of the oxygen fugacity (e.g. annealing in air or Ar-atmosphere) on changes in oxidation states and potential color changes are given in Figure .