I'd be real careful about sharpening that. For one thing, such crystals do not like chattering vibration along an axis, and it would be pretty easy to chip that edge, or fracture the crystal altogether. It is quite brittle, and has virtually no tensile strength, so you are not going to want to beat it against or with anything that isn't real pliable. The sharpest edge will likely be the result of longtiudinal fracture, much like an obsidian edge or in the style of knapping stones. It should be able to take an edge by ablation using diamond or silicon carbide, but such an edge will most likely be micro-serrated, which is not necessarily a bad thing.

If it has a good edge now, I wouldn't mess with it at all. If you do use it for slicing, try to slice rounded suraces rather than squared edges on anything with a brinell hardness of more than 40 or so.