Ok yes, they all contain something that's literally made of crystal, but I meant crystals in terms of the specific role they filled the early games (hence the quotation marks). The crystals in FF1, FF3, FF4 and FF5 are pretty much the exact same thing in every game, as far as I remember (though those are not the games I know the best, so I'm probably forgetting some details). So the crystals are not just any old magical item, they're specifically playing the role of a kind of magical guardian over the forces of nature, and often protecting the specific kingdom where they're located. Which means it's usually part of the story that the crystals will be under threat of being destroyed or taken, which will lead to some kind of worldwide disaster.
That is a story-telling trope they largely abandoned after FF5. Then in recent years they seem to be putting it back into all of their new games. They used in Bravely Default, they used it in FFXV, and based on the specific dialogue in this trailer, it sounds like they're doing the exact same thing in FF16.
I'm just wondering why they thought that was an idea they should resurrect. Not saying I have any problem with it or anything. It just seems a bit random. Maybe it's because they're trying to compensate for the fact that the gameplay has moved more and more away from the traditional series formula, so they're deliberately including more of the story tropes as a way of saying "see, it's definitely still final fantasy".