Corals surface roughness

Thomas, F. I. M. and Atkinson, M. J. (1997). Ammonium uptake by coral reefs effects of water velocity and surface roughness on mass transfer, Limnol. Oceanogr., 42, 81-88. 

The adhesive forces may also vary with the roughness of the particle. Thus, the adhesive force between the particles of coke shape [152] possessing microscopic surface roughness and a plane surface is smaller than that of smooth spherical particles of the same material. This is because contact between the coke-shaped particles and the plane is effected at individual points, which reduces the contact area and hence the adhesive force. Coke-shaped powders include coal and silica gel particles [153] as well as particles of dust and ashes formed in nuclear explosions in carbonate soils (coral reefs), when the soil particles are decarburized and acquire a flocculent shape [157] similar to that of coke. In general, powders of coke shape may be obtained by combustion of the volatile components from the particle surface. 

Red soft coral is also matt, but has a rough surface. It must be impregnated with resin in order to be comfortable when worn. The result is, like blue coral, a sheen rather than a high polish (Fig. 11.9). The resin used to impregnate blue coral and red soft coral sometimes contains a pigment to enhance or darken the natural colour of the coral (Fig. 11.6). 

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