A crystal sword may not be the most practical implement for combat, but in the fantasy world this is placed in, it is actually an amalgamation of steel that is enchanted with spell-bound blood. Just before being placed in quenching oil, after the blade's last heat treating. The steel is opaque during most of the smithing process, but in the final stages of forging, the folds of steel harden. At the molecular level, the steel rearranges itself; crystallizing to form into this semi- translucent material.