Trigonal crystallite

When a very narrow X-ray beam (with a spread of wavelengths) is directed on the center of a genuine pearl, all the crystallites are irradiated parallel to a trigonal axis and the result is a Laue photograph with 6-fold symmetry. In a cultured pearl the narrow beam will have an arbitrary orientation with respect to the crystallite axes (of the central core) and an unsymmetrical Laue photograph will result. (See J. Bijvoet et al., X-ray Analysis of Crystals. Butterworth (1951).)... 

Structure. As a general rule, carbon-based materials have similar microstructures consisting of a planar network of a six-membered aromatic-forming layered structure with sp -hybridized carbon atoms trigonally bonded to one another. The crystallite size and extent of microstructural order can vary from material to material (i.e., edge-to-basal-plane ratio), which has important implications for electron-transfer kinetics. 

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