The beauty of opals results from a densely packed, highly ordered arrangement of silica spheres with a diameter of several hundred nanometers. Such ordered nanostructures are typical examples of materials called photonic crystals, which can be formed by known microstructuring methods and by self-assembly. Opals represent a self-assembly approach to these structured media; such an approach can lead to novel materials for photonics, photocatalysis, and other areas. Although self-assembly leads to many types of defects, resulting in the surprising and very individual appearance of natural opals, it causes also difficulties in technological applications of opal systems.

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Reduction of metal salts with tetraalkylammonium hydrotriorganoborates in organic phases yields a very narrow size range of metal colloids of the elements of groups 6–11. Adsorbed on carrier surfaces, these colloids are highly active hydrogenation catalysts whose activity can be further increased by doping with low‐valent organotitanium compounds.

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