Amyloid stained by Congo red is traditionally said to show apple-green birefringence in polarized light, although in practice various colors may be seen between accurately crossed polarizing filters, called polarizer and analyzer. Other colors are seen as the polarizer and analyzer are uncrossed and sometimes when the slide is rotated. Previously, there has been no satisfactory explanation of these properties. Birefringence means that a material has two refractive indices, depending on its orientation in polarized light. Birefringence can change linearly polarized light to elliptically polarized, which allows light to pass a crossed analyzer. The birefringence of orientated Congo red varied with wavelength and was maximal near its absorption peak, changing from negative (slow axis of transmission perpendicular to smears or amyloid fibrils) on the shortwave side of the peak to positive (slow axis parallel) on the longwave side. This was explained by a property of any lightabsorbing substance called anomalous dispersion of the refractive index around an absorption peak. Negative birefringence gave transmission of blue, positive gave yellow, and the mixture was perceived as green. This explains how green occurs in ideal conditions. Additional or strain birefringence in the optical system, such as in glass slides, partly or completely eliminated blue or yellow, giving yellow/green or yellow, and blue/green or blue, which are commonly seen in practice and in illustrations. With uncrossing of polarizer or analyzer, birefringent effects declined and dichroic effects appeared, giving progressive changes from green to red as the plane of polarization approached the absorbing axis and from green to colorless in the opposite way. This asymmetry of effects is useful to pathologists as a confirmation of amyloid. Rather than showing 'apple-green birefringence in polarized light' as often reported, Congo red-stained amyloid, when examined between crossed polarizer and analyzer, should more accurately be said to show anomalous colors.

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Fourier transform infrared mapping of diamonds can reveal detailed information on impurities, with a spatial context. We apply this technique, combined with in situ isotopic analysis of carbon, to the study of cuboid diamond growth in a sample that exhibits some mixedhabit growth. While there has been some uncertainty in the literature regarding sectoral differences in nitrogen aggregation and subsequent platelet development, the data from this study appear far more conclusive. We show that despite nitrogen being concentrated in octahedral sectors, there is no detectable difference in the concentrationadjusted rate of nitrogen aggregation within octahedral and cuboid sectors. However, the resultant platelet development is significantly reduced in cuboid sectors compared to contemporaneously formed octahedral sectors. This finding has significant implications for the classification of diamonds using the relationship between their platelet intensity and the absorption caused by B centres. It means that cuboid diamonds naturally fall below the linear relationship that has been termed regular, which would lead to them being incorrectly interpreted as having experienced heating or deformation. The data also support earlier suggestions that large hydrogen concentrations in the diamond-forming fluid may be required for cuboid growth. We further suggest that high nitrogen and hydrogen concentrations are required for mixed-habit diamond growth, which might be the product of specific fluid chemistries that occur in reducing mantle environments.

MDS and PCA derivationMultidimensional scaling (MDS) and principal component analysis (PCA) solutions are derived using R (version 3.5.2 "Eggshell Igloo," released 20 December 2018) and RStudio GUI interface. Derivation methodology is outlined Data preparationAll δ 13 C data are rounded and truncated to two significant figures after their decimal (x.xx).All [N] data are rounded and truncated to one significant figure after their decimal (x.x). MDS is performed using δ 13 C data only, and data are formatted in a .csv file (MDSdata.csv) with two columns: (1) Locality and (2) δ 13 C. PCA is performed using δ 13 C and [N] data, and data for are formatted into a .csv file (PCAdata.csv) with three columns: (1) Locality, (2) δ 13 C, and Multidimensional scaling (MDS)Metric MDS is performed on all analyses with δ 13 C data. MDSdata.csv is imported into R. The

Diamond BZ270 derives from within a 30 km radius of the small Rio São Luiz, near the town of Juina. The sampling site for diamond JUc4 can be more precisely constrained to 261000 m E and 8708000 m N in WGS84 UTM-projected Zone 21S (Agrosi et al. 2017; Figure DR1). METHODS Micro-Computed X-ray Tomography Micro-Computed X-ray Tomography (Cnudde and Boone, 2013) was carried out using a Scyscan 1172 microtomograph, located at the University of Bari (Italy). A 45-kV X-ray source was used with a current of 218 µA. A total of 1200 absorption radiographs were acquired over a 360 ° rotation with an angular step of 0.3 °. Random movement of the vertical axis and multiple-frame averaging were used to minimize the Poisson noise in the projection images. Beam hardening was reduced by the presence of a 0.5 mm Al-filter between the source and the detector. The nominal spatial resolution for the resulting model was 4.75 µm. The raw data were reconstructed into two-dimensional slice images using the software "NRecon, Skyscan, Belgium". Corrections for the beam-hardening effect and ring artifacts were also applied during the reconstruction process. Micro-CT data were analyzed using the software "CT-analyser, Skyscan, Belgium". Cathodoluminescence Cathodoluminescence images were collected on a Centaurus detector attached to a Hitachi S-3500N scanning electron microscope (SEM; School of Earth Sciences, University of Bristol). Samples were carbon coated prior to imaging. Accelerating voltages were varied between 10 and 20 kV to obtain the best quality images.

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