An updated and expanded Type classification scheme for diamonds based on FTIR spectroscopic data

The Type classification of diamond is based on the presence or absence of nitrogen (N) and boron and the aggregation state of N-defects determined via Fourier transform infrared (FTIR) spectroscopy. Our understanding of different N-defects in natural, synthetic and treated diamond has expanded considerably since the historical Type classification was proposed ∼100 years ago. Consequently, important geological and gemological information is not included in the Type classification scheme. Moreover, there is no systematic approach to expressing the fraction of the major N-defects in the Type classification scheme. To address these issues, and standardize the classification of diamonds across different disciplines, we propose an updated Type classification scheme for diamonds based on FTIR spectroscopic data. The updated scheme includes four Tiers allowing classification at varying levels of detail depending on spectrum quality and/or the amount of information the user wishes to extract. This tiered scheme allows basic classification of diamonds using Tiers 1-3 and, as an optional step, more detailed classification using Tier 4. By differentiating the instrumental detection limit and practical quantitation limit of N-defects, rigorous criteria for Type I and TypeII diamonds, and new Type Ib (e.g., Type IbX and IbY) and Type Ia subgroups (e.g., Type IaA′ and IaB′), are established. Additional Tier 4 subgroups describe, platelets, CO2 inclusions, amber centers, and synthetic and treated diamonds (Type Ib–Ia diamonds with C-, A- and B-centers). A detailed description of these features and other types of point- and extended-defects observed in natural and synthetic diamond is given. Examples are provided demonstrating how the new scheme is used to classify diamonds such that useful geological and gemological information can be extracted from each Type designation. The effect of sample thickness, signal saturation in the two- and one-phonon regions, and spectral quality/resolution on the accuracy of calculated N-defect concentrations is evaluated. Moreover, issues assigning a specific Type to zoned diamonds based on individual FTIR spectra are discussed.