[color:"orange"]Where did I get this holy water from? Is it a reward from somebody? Is it only good for the money you can sell it for? [/color]
You get a holy water as a quest reward for returning the amulet to the imp hermit. You can use the holy water to make a permanent potion. For this you need alchemy extract level 4 and the appropriate alchemy potions.
To find out which alchemy plants you need for which potions, click here

Further about the crystals: no, they don't add a lot of resistance; but that's not the same as saying they're utterly useless. I did use the large shadow crystals with the shadow creatures, and I did notice the difference...
Also in after-game BF with mages doing a particular kind of damage they helped quite a bit (of course, at this point you have extra large crystals of just about each kind, so the added resistance is a wee bit higher...)

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