The “amber centres” and their relation to the brown colour in diamond

The amber centres are a series of similar near-infrared absorptions found in brown “amber”-coloured diamonds. These centres are found exclusively in crystals showing lamellae of plastic deformation (“brown graining”) and nitrogen in the form of A-aggregates, at least. They represent likely electronic transitions responsible for up to six zero-phonon lines and associated vibronic structures. Four different categories may be distinguished. The most common one is proposed to be caused by a “defective” A-aggregate.