Hi this is the section of my Chrono Trigger page which has The location of the sealed boxs in the game. You can only open these boxes with Marles pendent, and after the pendent is charged.

600 ad, 1 box, truce inn

600 ad, 1 box, guardia forest

600 ad, 1 box, guardia castle

1000 ad , i box, truce inn

1000 ad, 1 box, guardia forest