The Starglass is an item in Dark Chronicle capable of reversing time to a point of great importance to the user. In the main storyline, Max and Monica purchase one from Starlight Temple 100 years in the future in order to travel back through time with Lin and prevent Crest's death. It is used at the ruins of the lighthouse in Balance Valley, the site of Crest's death at the hands of Gaspard and the Death Ark. However, their intervention is still not enough to prevent the death of the future great sage.
One Starglass can be purchased for 300 Gilda from Starlight Items in the future Starlight Temple as a key item.
Monica's Starglass[]
Upon entering the Miner's Breakroom level of Zelmite Mine, Max is surprised by a familiar face from the future. With the Atlamillia gone, Monica used a Starglass to return to Max's time and aid him in the Forgotten Adventure to restore the Blackstone Railroad. The fact that she was able to go back to Max using a Starglass suggests that he meant something to her, especially since the point in time in which she appears is one she hadn't already experienced (a la Lin). However, lacking the red Atlamilia and a ready source of Starglasses, which can apparently only reverse time, Monica appears to be permanently in Max's time.