martensite

Martensite, named after the German  Adolf Martens (1850–1914), most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by diffusionless transformation. It includes a class of hard minerals occurring as lath- or plate-shaped crystal grains. When viewed in cross section, the lenticular (lens-shaped) crystal grains are sometimes incorrectly described as acicular (needle-shaped).