Optical Properties of Diamond: Type I & Type II Diamonds
Surface Luster of Diamond
Article Copyright © 2009 AllAboutGemstones.com
The surface luster (or "lustre") of diamond is described as adamantine, which means unyielding, inflexible, or having the hardness or luster of a diamond. The term adamantine describes the way light interacts with the surface of a crystal, giving off a somewhat greasy sheen.
The word luster traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance.
Fluorescence in Diamonds
Fluorescence is an optical phenomenon in which a diamond's molecules absorb high-energy photons, re-emitting them as lower-energy, or longer-wavelength photons.
Long Wave/Short Wave UV Cabinet
Diamond Fluorescence Under UV Light
Diamond types that exhibit the phenomenon of fluorescence radiate or glow in a variety of colors when exposed to long wave ultra-violet light, and give off a bluish-white, greenish or yellow fluorescence when exposed to the X-ray wavelength. Some diamond varieties, particularly Canadian diamonds, show no fluorescence, and appear dark when exposed to ultra-violet light or X-rays.
Type I & Type II Diamonds
As many as 99% of all natural diamonds are classified as Type I, and contain nitrogen atoms as an impurity, which replace some carbon atoms within the crystal lattice structure. These Nitrogen impurities found in Type I diamonds are evenly dispersed throughout the gemstone, absorbing some of the blue spectrum, and thereby making the diamond appear yellow.
There are also two subcategories (a and b) within each diamond 'type' (either Type I or Type II) that are based on a stone's electrical conductivity.
All Type 1 diamonds have nitrogen atoms as their main impurity. If the nitrogen atoms are grouped in clusters they do not necessarily affect the diamond's overall color, and they are classified as Type 1-A. If the nitrogen atoms are dispersed evenly throughout the crystal, they can give the stone a yellow tint, and are classified as Type 1-B. Typically, a natural diamond may contains both Type 1-A and Type 1-B material. Man-made synthetic diamonds containing nitrogen are classified as Type 1-B.
Type II Diamond Formation
Certain diamonds were formed under extremely high pressure for longer time periods. These rare diamonds have a lower nitrogen content, permitting the passage and reflectance of blue light, making them appear 'colorless' (D). Type II diamonds do not contain any detectable nitrogen, thereby allowing the passage of short-wave ultra-violet (SWUV) light through the stone. Natural blue Type II diamonds containing scattered boron impurities within their crystal matrix are good conductors of electricity, classifying them as Type IIb diamonds, while Type II diamonds that lack boron impurities are classified as Type IIa.
Cloud Inclusion Under UV - © AGS Labs
Type I UV Fluorescence - © AGS Labs
Type IIa diamonds are very rare, and some of the finest historical gemstones such as the Cullinan and Koh-i-Noor are both Type IIa diamonds. These Type IIa diamonds have a near-perfect crystal structure making them highly transparent and colorless, with very high thermal conductivity.
Some Type IIa diamonds can be found with pink, red, or brown coloration, due primarily to certain structural anomalies arising from "plastic deformation" which occurred during their formation.
Diamond Refraction & Coloration
Diamonds are "singly refractive," with a refractive index of 2.417. Diamond can exhibit pseudochromatic coloration giving the appearance of "color" without having any actual color in the mineral itself. This illusion of color is caused by the varying optics effects created by spectral dispersion, or "fire," and refraction.
Diamond Refraction & Light Dispersion
Diamonds can also exhibit allochromatic coloration which is caused by chromophores from the nitrogen trace impurities found within crystalline structure. It is this nitrogen component that produces the color of fancy yellow diamonds.
Diamond Color & Composition
Diamonds can occur in a wide variety of colors: colorless or white, blue, steel grey, pink, orange, red, brown, green, yellow, and black. All colored diamonds contain certain specific impurities and/or structural defects that cause their coloration, while chemically "pure" diamonds are basically transparent, and therefor colorless.
On To: Diamond Chemistry & Composition
Bibliography on Diamond Optics
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3. University of Bristol, Allotropes of Carbon . www.chemsoc.org
4. , Inclusions in Diamonds from K14 and K10 Kimberlites. www.sciencedirect.com
5. V R Howes 1962 Proc. Phys. Soc., The Graphitization of Diamond . www.iop.org
6. Nature Article, Growing Diamond Crystals .
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14. Smithsonian exhibit, Colored Diamonds . www.mnh.si.edu
15. GIA, Gemological Institute of America . www.gia.edu
16. AGSL, American Gem Society Laboratories . www.ags.org