New phenomena significant in relation to optical properties of diamond and to inhomogeneity of impurity distribution in synthetic diamond are reported. Evidence has been collected from (a) optical transmission at 430 nm, (b) Schlieren photography, (c) birefringence, (d) infrared absorption microscopy, (e) differential abrasion resistance patterns, (f) cathodoluminescence topography with analysis of polarisation of emission, and (g) X-ray topography with synchrotron and conventional sources; and is detailed and correlated for one specimen, supplemented by data from a second, similar specimen. The bulk of the specimens comprised material belonging to one cube growth sector, designated (001). They did not contain the seed, which had been polished away. The specimens were slab-shaped, bounded by a pair of polished surfaces parallel to (001), nearer and further from the seed, respectively. The principal feature of interest on both surfaces parallel to (001) was a “Maltese Cross” type of cathodoluminescence feature, radiating from a growth centre that had dominated growth in the direction [001] throughout the thickness of the slab (1 mm), entirely covering the (001) growth surface up to its maximum width of 3 mm exposed on the specimen surface remoter from the seed. The arms of the Maltese Cross brightest in visible emission (peak wavelength ≈ 525 nm) expanded radially in the cube diagonal directions ±[110] and ±[1 1 0]. Their cathodoluminescence emitted in the direction normal to the (001) surface was partially polarised with E-vector parallel to the radial direction in each arm. Less bright emission from arms pointing in the cube directions ±[100] and ±[010] was unpolarised in this [001] view. Application of techniques (a), (b) and (d) showed that the former, diagonal arms were at least 25% to 30% richer than the latter arms in nitrogen impurity present in singly-substituted atomic state, measured at a distance of 600 μm from the cross centre. Differences in impurity incorporation and optical properties are attributed to differences in surface structure on vicinal slopes of a low-elevation growth pyramid. Regarding polarisation of cathodoluminescence emitted in the [001] direction, the diamond structure fourfold screw symmetry normal to (001) provides the foundation for a growth step model that can account for unpolarised emission from the cross arms pointing towards ±[100] and ±[010], and the 90° rotation of partial polarisation direction between arms pointing towards ±[110] and those pointing towards ±[1 1 0].