bone black

bone black solid black material, largely carbon, produced by heating animal bones to high temperatures in the absence of air so as to drive off volatile substances. Finely divided bone black is useful as a pigment; bone char, a similar material, is an important source of activated charcoal for use in refining and decolorizing sugar.

black shales Although many sedimentary rocks are coloured various shades of grey, truly black examples are rare: coal and black shales are the only two common types. Both these rocks owe their colour to a high concentration of organic matter; a content that makes them the most economically important of all rock types as they are the source of all the world's fossil fuels. The organic matter of black shales tends to comprise a high proportion of complex organic molecules derived from plankton which, when buried deeply enough (typically to depths of 3–4 km), break down to form oil; black shales are therefore often petroleum source rocks.

As well as their economic importance, black shales are also celebrated for their exquisite fossil content. Black shales tend to accumulate slowly in deep, poorly oxygenated, stagnant seas and lakes. These conditions are unfavourable for bottom-living creatures but they have little affect on the animals swimming around in the upper water column. On death, such animals (e.g. ammonites, fish, whales, and marine reptiles) sink to the sea floor where they are buried and fossilized in beautiful detail: fish scales and bones are commonly pre-served in place, undisturbed by scavengers, and even skin impressions are not uncommon.

P. B. Wignall