Yeah, they're all the same. Only one pendant was made by Melchior, and this is confirmed by the fact that only Schala could activate the Mammon Machine with it; I'm sure Queen Zeal would have jumped at the chance to do it herself as soon as possible if she weren't required. Also, several Zealians remark that Marle's is similar to Schala's (similar, as in mistaking it outright for the princess's. Melchior does this too).
From somewhere in Cross, I suddenly believed the crackpot notion that Lucca forged the Astral Amulet. Although Lucca is certainly capable of building incomplete Time Eggs, it is said otherwise that Schala gives Kid her pendant.
Thus, apparently, Dreamstone...can have time-altering effects. Or at least amplify them? Wha? We have a choice here after researching this a bit. Either Dreamstone has the power to alter time, or Schala/Kid/Marle use it to confuct and amplify a power that has time altering abilities. Ok, #2 seems correct; Lavos' emanations were notorious for causing disruptions and half-baked gates. Since he still resided within the planet during the events of Chrono Cross, perhaps Kid's pendant only used 'Mammon energy,' as I call it, to have the effects of rewinding time to a safer position.