Optical properties of Diamond

 Optical Properties

Effects of Light on Diamond

Absorption Spectra

The absorption spectra of most diamonds may be divided into two main groups. Diamonds of the Cape series, which fluoresce with a blue light, and have a body colour varying from colourless to yellow. In this group the strongest absorption line is at 415 nm, varying in strength with the depth of the body colour and often noted in colourless specimens. Associated with it are lines at 478 nm (often the only one visible), 465, 452, 435 and 423 nm.

Diamonds in the second group have a brown, greenish yellow or green body colour and show a green response to UV. Absorptions include a strong narrow line at 503 nm with weak lines at 537 and 495 nm. Blue and green fluorescing crystals may also show the 415 nm line.

Some colourless, bright yellow and brownish yellow diamonds, with a yellow fluorescent glow under UV, may show no discrete bands at room temperature except for, maybe, a weak line at 415 nm. Blue (type IIb) diamonds absorb slightly in the red which is difficult to observe with the hand spectroscope. Fluorescence is often seen to be banded or sectorized!

 

Ultraviolet

Under LW the response may be blue, green or yellow, sometimes a red- dish glow. The strengths of the response varies from weak to a strong sky-blue. Fluorescence response has been the basis of a number of ingen- ious methods for recording individual diamonds so that they can be traced in the event of loss.

Diamonds fluorescing a bright blue do invariably, however, show yellow phosphorescence varying in strength with the strength of the blue glow. This combination has been recorded only in diamond. Some pink diamonds show an orange fluorescence with persistent orange phosphorescence. The spectroscope will show a bright line at 575 nm weaker bands at 586, 598 and 618 nm. The bright line at 575 nm is sometimes accompanied by an addi- tional bright line at 537 nm. These bands have been reported from diamonds coloured by atomic bombardment.

Under short-wave ultraviolet light (SWUV) the fluorescent effects are in general similar to those observed in LW but the response is markedly weaker. Under X-rays most diamonds show a rather uniform bluish white glow; the exceptions are those diamonds which show a yellow glow under UV and show a similar glow under X-rays but this is not always so.

 

Robertson, Fox and Martin (1934) found that some diamonds were more transparent to UV than others. This was made the basis of a classification of diamonds into two types – type I and type II.

Type I diamonds are transparent down to about 300 nm while type II are much more transparent and pass UV down to about 225 nm. In 1952 Custers subdivided the type II diamonds into type IIa and type IIb. The type IIa are said not to phosphoresce when irradiated with SWUV, while the type IIb when similarly irradiated shows a bluish phosphorescence and will also conduct electricity. In these type IIb diamonds sometimes the afterglow is red, and this is so with the famous Hope blue diamond. The electrical effects shown by type IIb diamonds are due to the presence of boron atoms.

During 1959 Kaiser and Bond found the reason for the difference in the absorption of UV to be due to free nitrogen in type I diamonds, and this type has been divided into two sections by Dyer and others in 1965): type Ia in which the nitrogen has been shown to be in groups of two or more atoms, and type Ib in which the nitrogen is dispersed in substitutional sites in a paramagnetic form. Any nitrogen in type II dia- monds is in extremely low concentrations. There is some evidence that well-formed diamond crystals depend to some extent on the presence of nitrogen; and, further, it is noticeable that large diamonds which are found do not show any good crystal form and are almost surely type II diamonds.

 

X-Rays

The transparency of a substance to X-rays is a function of the atomic mass of its constituent elements p. The higher the atomic mass of the ele- ments the less transparent it is to X-rays. Diamond is pure carbon (atomic mass 12) and is notably transparent to X-rays, whilst zircon which has the heavier zirconium atom (atomic mass 91) and silicon (atomic mass 28) and indeed all colourless stone and glass which simu- late diamond are much more opaque to X-rays.

 

Inclusions and Light

To be of the best quality, diamond must be clean (free from conspicuous inclusions). These imperfections may, however, be so small as to be unde- tectable under a 10 lens (the stone is then said to be flawless, internally flawless or loupe clean) or the flaws may be so obvious so that the beauty of the stone is affected. A single large imperfection in the wrong place in a cut diamond may be reflected from all the back facets so that to the eye the stone may appear full of inclusions.

 

A most important study of the internal features of diamond has been made by Gübelin who has identified in diamond inclusions of graphite, hematite, magnetite, garnet, diopside and enstatite among others. Diamond crystals can often be seen. Transparent inclusions may appear as black spots when light is totally reflected at their surfaces.