Not much is known about Xana Crystals. There are apparently eight of them throughout Radia, though they are not really well known or protected by the people of Radia. Many of them are sealed away and remain a mystery to most. A couple may or may not be well protected by the military...we'll see! One thing's for certain: they are what the Gyth are after, and they are connected to magic in some way.
There are smaller Xana crystals and Xana shards that are collected throughout the game or embedded in weapons/armor for magic bonuses. These are not the same as "the" Xana Crystals.
Another Crystal was in the sealed Goddess Chamber of Roth Temple, and only Richard seems to have access to it due to the Matrimony Bonds taken by himself and his wife (though neither knew anything about the Crystal or its value).