If you played FFXI, Dark Crystals were used on items to make them "decay", while Light Crystals were used to "restore" or fix things, such as fishing rods. I only came across one Light Crystal in the whole game, and that's when I defeated a Crawler out in Sarutabaruta. I have always wondered why there are no Light or Dark Crystals to be found in this game, like for crafting. I know that in 1.0, I got a crystal which had its energy drained from it. I forgot what it was called, though. It's un-something, and I can't remember how it was spelt. Got that from fighting a monster in The Black Shroud in 1.0.