What is the use of a marble square ruler

Marble square ruler is mainly used for measuring the flatness, verticality, and straightness of workpieces. The main mineral components of a marble square ruler are calcite, plagioclase, a small amount of olivine, biotite, and trace amounts of magnetite. It has a black color and a structural code. After billions of years of aging, the texture is uniform, the stability is good, the strength is high, and it can be maintained under heavy loads. Suitable for industrial production and laboratory measurement work.