Masamune is also the name of the most powerful two-handed sword in Chrono Trigger. (Not a katana, strangely. The strongest katana is the Rainbow.) In the game, Frog is the only one capable of wielding it.

Created from the same stone used in the Mammon Machine, the Dreamstone, by Melchior in 10000 BC, Masamune was only a knife until used to destroy the Mammon Machine, when it turned into full sword glory. It is unknown whether it was broken right there or somewhere between 10000 BC and 600 AD: What is known is that the blade and the hilt were separated by the time Crono and his party got to it.

The sword houses two wind spirits, called Masa and Mune. (...) They defend the blade, waiting for those that are worthy to take it and try to fix it. However, only one being can use the Masamune to its fullest glory: the holder of the Hero Medal.