Term for opaque or opaque flat materials and layers (lat. obacus = cloudy, blurred; opacity = measure of turbidity). This is mainly used for strongly absorbing minerals (ores) that appear opaque in visible light. Occasionally, opaque is used in a positive way for the colour depth of a red wine in the context of a wine appeal (the darker the colour = opaque, the "more beautiful" the wine). A related but negative term is blind.
A special sedimentary rock (loam marsh) is called Opok (confusingly also opaque) in Styria (Austria) and Slovenia, but has nothing to do with the above term.