the masamune is made of dreamstone, so it has certain... mythical properties. it was the ability to REFLECT the heart of the wielder. when pure hearts like glenn and cyrus used it, it appeared as a powerful two-handed sword.
in chrono cross, however, we know that it is a crimson, spiked sword that causes it's user to become psychotic.
i always assumed that post-trigger but pre-cross someone truly and completely evil obtained the masamune, and by reflecting that persons heart it was transformed into it's cross form. (i think the chrono trigger psx remake ending showed the shadowy figure to hint this.) then, the masamune remained in it's evil form until someone pure of heart would be able to override it's evil, turned into once more into a reflection of goodness. karsh, dario, radius, and whoever used it in cross weren't pure of heart enough to turn the blade once more; the good in their hearts was not enough to override the evil contained in the blade. however, once dario was defeated and freed by serge and company, serge's heart was pure enough to cause the masamune to once more reflect goodness. however, because it reflects the user's heart, for serge it was a swallow instead of a sword. when doreen joined her brothers, it powered up the masamune to become the mastermune, an even more powerful weapon.
