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Martensite, named after the German metallurgist Adolf Martens, is a class of hard minerals occurring as lathe- or plate-shaped crystals. When viewed in cross-section, the crystals appear acicular (needle-shaped), which is how they are sometimes incorrectly described. The crystals are a body-centred cubic (BCC) form of iron and carbon, and result from the rapid cooling of austenite during quenching. In the 1890s, Martens studied samples of different steels under a microscope, and found that the hardest steels had a regular crystalline structure. He was the first to explain the cause of the widely differing mechanical properties of steels. Martensitic structures have since been found in many other materials.