I seriously cannot remember any names or parts of the title, I apologize. I first watched this anime at least 10-15 years ago (most likely between 2001 and 2008) online, when even the last few episodes hadn't been fully translated from Japanese to English at the time. It had subtitles, and may have been a fan translation rather than an official airing. It's a 'modern' fantasy where most of the world isn't aware of magic existing, I believe. This was also not a long series, with only about 12 episodes or so.
The main girl is 'prophesied' to save the world, so she goes to different ruins (?) where they are trying to gather these gem like fragments studded into silver-colored shields that will float around and protect the main girl. However, when all the fragments are collected, it turns out the girl and her friends were lied to, and when the gems activate they turn the girl into a gigantic monster who tries to destroy the world. She is trapped inside the monster. Her friends stop her and defeat the villain who was pulling the strings in the end (because of friendship and love? - one of those cheesy ones).
The main character is a girl, but there is at least 1 other girl and 1 boy in the group of teenagers the story focuses on. I'm pretty sure they were in high school and wore generic uniforms (blue or grey?). But they didn't spend much time in class or with their classmates (or it wasn't important enough to the story to remember). The main group never exceeded 5 characters, and I really only vaguely remember the main three: The main girl was more petite and had short to medium-length hair (not blonde or red), and had a quieter and more shy personality. The guy friend was very tall and also fairly quiet, while the friend who was a girl was more friendly. I think the guy had short, greyish hair, while the girl had longer hair. There wasn't a romance, but it did feel like it hinted the guy and girl friends were interested in each other. The MC had been alone before being told she was special, and so friendship is important to her.
This was one of those series that had pacing issues, where even though there's a limited number of artifacts (around 12 or so), rather than space out 1 artifact per episode, most of the series is filler and then in the last 2-3 episodes they get all of the artifacts very quickly and rush to the climax.
There's an episode where they go on a picnic, keeping the thermos and the picnic basket in an interdimensional space. There may have also been a floating island/castle (where some kind of prophet/wizard/diety? resided) and a battle in the sky near the end, right before they collect the last of the artifacts.
I could have mis-remembered a few things above, but I do remember the outro music being fantastic!