HomeColour in Diamonds

- Colour in Diamond
- Description of the importance of crystalline imperfection
- Diamond type

Knowledge of the various diamond types aids a gemmologist in the understanding of diamond synthesis and treatments.

Most natural diamonds contain some kind of defects and/or impurities within the crystal structure. Based on these defects and/or impurities scientist use infrared spectrometry to differentiate the types of diamond. There are Type I and II. Within each type they are further divided into Type Ia, Type Ib; Type IIa, Type IIb, Type IIc. Within Type Ia it is again further subdivided into Type IaA, and Type IaB. A gemmologist should remember that most diamonds will compose of diamond characteristics of two or more types and/or subtypes, e.g. most natural Type Ib diamonds have a type IaA component. (Tay Thye Sun, Singaporean Gemologist, June 1999)

Type Ia=contains nitrogen in clusters (aggregates); type IaA contains predominantly A-aggregates (pair of nitrogen atoms, forms at lower geological temperatures), type IaB  contains predominantly B-aggregates (four nitrogen atoms surrounding a vacancy, forms at higher geological temperatures). Many type la diamonds contain similar amounts of A- aggregates as well as B-aggregates and are then called type laA/B. In these stones one can frequently detect a certain amount of N3 centers, which cause the light yellow colouration of "cape" diamonds.  Nitrogen may also be present in platelets which have a large extent and low thickness and which represent probably a structure of carbon and nitrogen atoms. This comprises about 98.9% to 99% of natural diamonds. Type IaA diamond is identified by peaks at 1225 and 1180 cm-1; Type IaB diamond by 1130 and 1180 cm-1

Type Ib=contains mostly isolated substitutional nitrogen (i.e. one nitrogen atom substitutes one carbon atom). About 0.2% belong to this type. Identified by peaks at 1344 and 1130 cm-1 in IR spectroscopy. This is due to the single substitution of N.

Type II diamond do not contain any significant amount of nitrogen. About 0.1% belongs to this type. These diamonds are very transparent in the SWUV range down to approximately 230nm

Type IIa=does not show any impurity-related absorption in the UV, visible or infrared parts of the spectrum (the optically most transparent diamonds).

Type IIb=contains boron as an isolated substitutional impurity, is therefore electrically conductive and always has a gray to blue colour. Stones with a very low boron content may appear near colourless.

Type IIc=type II diamonds which contain hydrogen as a substitutional impurity with a dominating absorption around 2900cm-1 in the infrared.

A certain Type III diamond or also called Lonsdaleite, a hexagonal allotrope of diamond and a very rare type of diamond that is found in meteorite mentioned in Diamond Dictionary p. 305 2nd edition is a mistake. Lonsdaleite is an allotrope of carbon. With a hexagonal crystal structure it cannot be a diamond type. (Personal communication with Tay Thye Sun, 2006) The mechanism and major influences of imperfections on colour and electrical properties Absorption characteristics Spectroscopy

UV/VIS/NIR spectroscopy: with this method complete spectra can be obtained in the range of 190-1000nm. It is basically measured how much light is absorbed or transmitted at what wavelength by a substance (here a gemstone) and this is then plotted in a graph . The spectral curves obtained give a lot of information about the colour of diamonds. Most colour centres (defects in the crystal lattice which cause colour) are detected by this technique. (http://www.gemlab.net/publication1.htm)

Liquid nitrogen immersion: this is a technique, where a diamond is held in a special cell immersed in liquid nitrogen, which has a temperature of –196°C. The extreme cooling is necessary to ensure the highest possible resolution of all absorption-features in UV/VIS/NIR- and Raman/Photoluminescence-spectroscopy. Rarely, liquid helium (-269°C) instead of nitrogen, is employed.(http://www.gemlab.net/publication1.htm)

FTIR or IR spectroscopy: absorption is measured from 800-25,000nm; this is especially useful to determine diamond types and/or impurity concentrations. This is important because certain treatments are only possible for certain diamond-types. The presence or absence of nitrogen hydrogen or boron as impurities defines the diamond-types (http://www.gemlab.net/publication1.htm)

Raman-Photoluminescence: a low-power laser of various wavelength (e.g. 405, 514, 532nm) is focused on a gemstone. Optical centres are excited by the laser and emit a measurable amount of luminescence. These luminescence phenomena appear as peaks in the spectrum. Photoluminescence detects certain colour centers with much more sensitivity than other spectroscopic methods and it is especially useful for synthetic and HPHT treated diamonds.(http://www.gemlab.net/publication1.htm)

Raman-(Micro) Spectrometry: this is a back-scattering technique where a low-power laser of various wavelength (e.g. 405, 514, 532nm) is focused on a gemstone and some of this laser-light is shifted to a longer wavelength and re-emitted by the tested sample. This so called "Stokes radiation" appears as peaks in the spectrum. These peaks are characteristic for the substance tested. The Microspectrometry technique is useful to identify most inclusions in gemstones, which do not necessarily have to reach the surface. Here, the laser is focused through a microscope and can identify inclusions as small as one micrometer (= 0.001mm).(http://www.gemlab.net/publication1.htm)

16.5.3 Use of UV radiation and X-Rays
16.5.4 Fluorescence
16.5.5 Phosphorescence
16.5.6 Cathodoluminescence

This technique is somewhat similar to Photoluminescence, but instead of a laser beam, an electron beam is being used to excite the optical centres. Cathodoluminescence can be either used spectroscopically or as an observative technique where the colour and the pattern of the luminescence can be observed. This is especially useful for synthetic diamonds, which show their characteristic growth zones in cathodoluminescence much more clearly and reliably than in regular fluorescence to longwave-/shortwave ultraviolet light. (http://www.gemlab.net/publication1.htm) 

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