I've read that cloud-to-ground lightning can create fulgurites. Does ground-to-cloud lightning also create crystals within the earth as it surges through the minerals?
I would assume that any lightning bolt connecting to the ground would be capable of producing a fulgerite in the soil, regardless of the direction that the bolt "flows". Fulgerites are caused by the extreme heat of the lightning bolt, which fuses the soil (sand mainly) together into a type of glass. Since the extreme heat of a lightning bolt also causes the sound of thunder, it's safe to assume if you hear thunder from a ground to cloud strike, it can also cause a fulgerite.
Most ground to cloud lightning seems to flow from the highest points of very tall objects, such as sky-scrapers, radio towers, etc. Since the best fulgerites are formed in sand, it may be very hard to find a fulgerite caused by ground to cloud lightning. Perhaps on a sandy mountain peak?