Have you ever had a crystal break? If course you have. And I bet you can tell me exactly which ones and in which order and how they broke. It's devastating, I know. There are MANY answers to the questions of why and what to do with the pieces. Hold your stone and follow your heart when deciding what to do with it. As with anything else in your metaphysical practice, there's no wrong answer.
Some people feel like stones break when they have fulfilled their purpose. As a sort of offshoot of that they say that sometimes the stone just takes on too much as it's doing its job (like if it's meant to remove negativity, maybe it breaks because there was more negativity than it could handle). Going back to what I say about people coming and going from our lives as divine timing dictates, I kinda feel like this is more why my stones break or get lost. And I suppose it's only responsible of us to acknowledge that sometimes we drop shit and it breaks.
What to do with them...the most popular answer is bury them. Return them to the Earth, they say! You have my permission not to do that. I have buried some stuff. it's a valid option for sure. But you bought these and love these and maybe you're not done with them yet. That's fine.
You can change their purpose. If it was small maybe it's time to have it made into jewelry. Maybe you're drawn to give one away. Balancing the energy by putting one in each hand can be a really special way to meditate, especially if you incorporate the reiki flow of your different hands. Display them as two different pieces. And, you officially have my permission to glue it back together if your little heart desires!