The Yellow Color Center and Trapped Electrons in Beryl

Investigations continue into the cause of yellow color in beryl. The generally accepted explanation is that it is related to the charge-transfer between Fe³⁺ ions substituting for octahedral Al³⁺ ions in the crystal and the surrounding oxygen ions. 
 

Using electron paramagnetic resonance (EPR), an absorption technique that uses magnetic fields to measure transition energies in atomic defect centers, this study contradicts the accepted interpretation, proposing an alternate model.
 

Although a strong EPR signal from octahedral Fe³⁺ ions can be found in beryl of all colors, including colorless, the study determined that Fe³⁺ ions substituting for Al³⁺ at the octahedral position cannot be the cause of the yellow color. There is, however, a signal unique to yellow beryl that comes from Fe³⁺ ions at a tetrahedral site. 
 

The author proposes a simple model describing the oxidation and reduction processes involving iron electrons trapped in the crystal structure to explain the creation and decay of the yellow color. This study discusses reinterpretations of other experimental findings that lead to different explanations for beryl’s colors.