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Type, color and clarity are three major parameters, which determine suitability of diamond for HPHT treatment. Clarity is the major limiting parameter. Low clarity diamonds with many inclusions may considerably stimulate internal graphitization and development of cracks during HPHT annealing making HPHT treatment useless. Diamond type is a technological rather than commercial parameter of diamond. It is a key factor for determination the HPHT treatment conditions, which could allow to achieve desirable final color. In this chapter, natural occurrence of diamond types and the methods of their recognition are discussed. Diamond color is of primary commercial importance. Initial color serves as the reference point for the evaluation of the improvement of diamond’s appearance and the increase in its price after HPHT treatment, as well as the effectiveness of the HPHT treatment procedure. Typical colors of natural diamonds used for HPHT treatment are discussed. A particular attention is paid to natural diamonds of brown color, which are the most common starting material for commercial HPHT treatment.

Technological principles, instrumentation and parameters of HPHT annealing used for commercial treatment of natural diamond are discussed. The emphasis is made on the description of design of high pressure cell and the methods of measurement of pressure and temperature during HPHT treatment. The preparatory procedures the diamonds undergo before HPHT annealing are discussed in detail. These procedures comprise selection of diamonds, their pre-shaping, cleaning and initial characterization. Examples of failure of HPHT treatment caused due to neglecting to follow the technological protocol are presented. Basic processes of the defect transformations and the color changes in brown diamonds of different color intensity and different type during HPHT treatment summarized and discussed. Commercial aspects of HPHT treatment, including the present state of the market of yellow-green and pink-red treated diamonds, cost of HPHT treatment and the final prices of HPHT-treated diamonds, are discussed.

Visual luminescence excited by UV light and luminescence imaging are discussed as simple and effective techniques for preliminary characterization of diamonds and as methods of recognition of HPHT-treated diamonds. Since all natural diamonds fluoresce under UV excitation, the visual luminescence technique is a common method applicable to diamonds of all types. The change of luminescence color after HPHT treatment and the formation of characteristics luminescence patterns in HPHT-treated diamonds are discussed. It is shown that the enhancement of luminescence on macroscopic defects and the concentration of luminescence intensity on the facet junctions of cut diamonds (“fluorescence cage” effect) are very characteristic features of HPHT treatment, which are particularly prominent in type I HPHT-treated and irradiated diamonds.

Numerous spectroscopic features characteristic of HPHT-treated diamonds are discussed. Many of these features are used as indicators of HPHT treatment. In this chapter, we summarize the data on all the optical centers throughout the UV-Vis-IR spectral range, which undergo changes during HPHT treatment and which are used for recognition of treatment. Characteristic combinations of optical centers observed in spectra of HPHT-treated diamonds are discussed. It is shown that none of the great many optical centers can be used alone for the reliable identification of HPHT treatment and only detailed analysis of many optical centers can allow a conclusive decision. Optical spectroscopy is discussed as the most reliable method of recognition of HPHT-treated diamonds.

In this chapter the main parameters of HPHT treatment are discussed. They are temperature, pressure and time. Temperature of HPHT annealing is the most important parameter, which determines the rate of the defect transformations in the diamond lattice and, consequently, the final structure of the treated diamonds. The range of temperature used for HPHT treatment is discussed in detail. The influence of pressure on the process of defect transformation during HPHT treatment is discussed. It is shown that pressure during HPHT annealing may vary considerably yet remaining in the range of graphite-diamond phase transition. Since kinetic of many defect transformations during HPHT annealing is slow, the duration of treatment is taken into account as an important parameter too. The differences of long-term and short-term HPHT treatments are discussed. High temperature short-term HPHT annealing is shown to be the most effective way of commercial HPHT treatment. Temperature–pressure-time parameters of HPHT treatment are compared with these of natural HPHT annealing the natural diamonds experience in earth. Color transformations in diamonds of various initial colors HPHT-treated at different temperature/pressure/time parameters are discussed. A particular attention is paid to the color transformation of brown diamonds. Optimal parameters of removal of brown color are discussed.

In this chapter, characteristic transformations of impurity-defect structure of diamonds during HPHT treatment are discussed. Detailed consideration of the processes of nitrogen defect aggregation and dissociation is given. Stimulating action of vacancies and nickel in the nitrogen-defect transformation processes is discussed. It is shown that the understanding of the nitrogen defect transformation processes is a key to recognition of HPHT-treated diamonds. Color transformations of natural diamonds of different types during HPHT treatment are discussed in detail. Special attention is paid to discoloration of brown diamonds and achievement of red/pink colors. It is shown that the transformations of the nitrogen-related defects and vacancy aggregates are the major processes determining the color change in HPHT-treated diamonds. Typical defect transformations during commercial HPHT treatment are discussed.

In this chapter, major structural features and macroscopic defects of HPHT-treated diamonds are considered. The most prominent of these are graphitized inclusions, cracks, surface modification and areas of internal stress. Behavior of inclusions during HPHT treatment and their impact on the clarity of HPHT-treated diamonds is discussed. It is shown that in most cases the HPHT-induced structural features of treated diamonds are similar to those observed in natural untreated diamonds and alone they cannot be used as reliable indicators of treatment.

High-temperature and high-pressure treatment of diamond is becoming an important technology to elaborate diamonds. This is the first book providing a comprehensive review of the properties of HPHT-treated diamonds, based on the analysis of published data and the work of the authors. The book gives a detailed analysis of the physics of transformation of internal structures of diamonds subjected to HPHT treatment and discusses how these transformations can be detected using methods of optical microscopy and spectroscopy. It also gives practical recommendations for the recognition of HPHT-treated diamonds. The book is written in a language and terms which can be understood by a broad audience of physicists, mineralogists and gemologists.