If I'm not mistaken it's in the Psionics Handbook. From what I've read (not including the PsiHB) Crystal Armor is something that the Gith make/use. I don't think they care for it getting into non-Gith hands. They're, from what I've picked-up on the boards here, nearly as rabid about this as they are with thier Silver Swords being 'lost'.
Considering the difficulty in making anything from a gemstone (and not having it break the first time used) I would say that anything made of crystal would have to be magically reinforced (like Ironwood or Glassteal)
Game wise I might consider some kind of reaction modifier (ie, you sure look purty in your big shiny armor) but I probably wouldn't make it any more ... armored I guess.
Oh, and I don't think I've ever seen actual rules for it, even in the Psionics Handbook. The Psionics Handbook DOES mention making plenty of Psionic Items out of crystal (including weapons and armor I believe) but it seems to be a flavor thing and not a game mechanics thing.
Heavily Psionically enhanced items tend to be part or all crystal. The price is included in the gp cost of the item, the difficulty in working it in the creation time, and the magical reinformcement in the enchantment. So a +2 crystal breastplate is just the same as a +2 breastplate apart from being made of crystal. It still costs 8000gp, provide +7 AC, etc. Major difference is that Druids can use it.
The armor in question was for a druid, so while cool, Nephelium won't suit my purposes for this exercise. No rules for crystal, as a component material? Guess I'll just fake it.
Well, if you're looking for an alternate material for druid armor... there are rules for shell armor somewhere, as well as pearl armor... just forget where it is off the top of my head.