Tephroite is a non-metallic manganese silicate mineral with the formula, Mn2SiO4, that is one of the unusual minerals found at the famous mines of Sterling Hill and Franklin, New Jersey, USA. It can also be found in England and Sweden.
Tephroite has a hardness of 6 and a specific gravity of approximately 4.1, which is heavy for non-metallic minerals. Its name comes from the Greek tephros - "ash gray"- for its color. It can also be found olive-green, greenish-blue, pink or brown.