However, when it comes to a broken crystal, the feng shui rule will be the same as to anything that is broken - you either fix it or let it go.
I assume that you are asking about a natural quartz crystal, which is most powerful for feng shui use, not a faceted lead crystal.
Use your own judgement depending on the severity of the damage to your feng shui crystal. If there is little damage, it might be ok to leave it as it is. Some people use glue to put the broken pieces of the crystal together, while some decide to discard the crystal.
If you decide it is best to let the crystal go, the most beautiful and respectful way it to return the crystal back to earth / bury the crystal back in soil. You can do that in the park, in the forest, in your own garden or even in the pot soil of your favorite home plant.