Fluorescence (7.41)
Fluorescence (a variety of luminescence) is the light radiation due to a reaction of the material atoms when exposed to incident light occurring in 25-35% of natural diamonds. The diamond absorbs photons, which is the fundamental part of the electromagnetic energy we call light, at one wavelength and emits it at a longer, lower-energy wavelength.
During crystal formation some 1 to 3.5 billion years ago, trace elements, commonly nitrogen and sometimes boron, get caught up in the lattice causing defects, (also known as fluorophores). The arrangement of these trace elements in the crystal lattice is critical in the cause of fluorescence such as the N3 clusters formation (see image below). Once they are exposed to long-wave ultraviolet light, the electrons of these defects transform into an excited state instantaneously emitting blue, yellow, green, white, orange and sometimes even pink photons.
The above image is courtesy of: plumbclub.com
Fluorescence is also a key indicator in identifying lab-grown diamonds in conjunction with diamond types: IaA, Type IaB, Type Ib, Type IIa, and Type IIb, discussed in the upcoming subsection 8.04 “Lab Grown Diamond Identification”: https://gemgate.com/blogs/education/synthetic-identification-8-04?_pos=9&_sid=0f359b7cf&_ss=r
The above image is courtesy of: https://www.gia.edu
Instead of the common N3 clusters, single isolated nitrogen atoms in the type the very rare (.1% in natural) Ib, can occur in natural diamond, sometimes causing an orangey-yellow fluorescence as well as phosphorescence, (a short-lived glow after the long-wave ultraviolet exposure is removed). In high levels, two adjacent nitrogen atoms, A-aggregates or four adjacent nitrogen atoms, B-aggregates, can minimize the intensity of fluorescence. For this reason, the majority of type Ia natural diamonds have inert fluorescence even though the nitrogen content may be high.
Phosphorescence is a light emission type when a substance absorbs the light-energy and slowly re-emits it as a visible glow, after the energy source ends, unlike fluorescence which stops immediately. This delay items, happens since electrons are get trapped in an energy state and is released gradually, perhaps for a few seconds, minutes, or sometimes even hours. In the case of natural Type IIb diamonds they may fluoresce often exhibiting a distinct blue glow and sometimes reddish, under UV light due to their boron content. they are also notoriously known for their strong blue to greenish-blue and red to orangey-red phosphorescence like the famous Hope Diamond's red afterglow. While some might show weak blue fluorescence, their unique, often long-lasting phosphorescence is a hallmark characteristic of this rare diamond type.
Above image is courtesy of: labdiamondsreviews.com
Caution: Shortwave ultraviolet light sources which are many times used in gemstone and lab-grown diamond identification requires eye protection.