This was one of the questions that was never answered in Chrono Trigger. Originally I had assumed that after Schala was lost in time, it somehow ended up in the Guardian family's possession, but when Chrono Cross seemed to contradict that theory. Of course, since it is from a game that involves time travel, i suppose the possibilities are endless.
Not to mention Chrono Cross involves two specific and suggests infinitely more dimensions there then more pendants to account for. Which is a greater headache than is necessary.
Let's just stick with the BnG phrase: "I hate time travel."
Yes. Let's just accept that and move on.
so that pendant has a magical charge, which will allow her to heal Crono, yay!