Crystal Ember: Found in the Duke's Archives. In the room with the staircase activated by the lever to get to the gardens, go behind the bookcase and open the chest to get the ember. Once you get the ember, bring it to the giant blacksmith in Anor Londo.
NOTE: Alternatively, you can get a Crystal Halberd from the mimic at the beginning of Anor Londo.
To ascend a weapon into a crystal ascension, you need to have any weapon at a +10 through standard path, then you take that weapon to the giant blacksmith and have him ascend it with a titanite chunk. You need 9 titanite chunks and a slab to get to +5 Crystal weapon. Refer to the lightning weapon achievement for methods on obtaining titanite chunks and slabs.