Term for opaque or light-impermeable flat materials and layers (Latin obacus = cloudy, blurred; opacity = measure of cloudiness). This is mainly used for highly absorbent minerals (ores) that appear opaque in visible light. In some cases, opaque is used in a positive way to describe the depth of colour of a red wine (the darker the colour = opaque, the more "beautiful" the wine). A related but negative term is blind. In Styria(Austria) and Slovenia, a special sedimentary rock (claystone) is called opok (confusingly also opaque), but has nothing to do with the term.
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