Opok is a soil type found in South Styria, Austria described as marly, a very fine-grained calcareous material (grain size 0.002 to 0.063 mm) consisting of clay and silt sediments, that was deposited in the sea. This grey rocky clay composed of clay silt [sediment] is, if it is not weathered, known colloquially as ‘Opok’. Opok can be either brown if it has a high sand content, or bluish if it contains higher percentages of clay.