Often you have a special stone, always on or near you, and it simply disappeared. This means that the stone has completed its work with you and has gone to someone else. who has need of it, or simply has gone to 'rest'.
When a stone has 'disappeared ' one very often finds you come into contact with a new stone quite soon afterwards, may it be you walking past a shop that sells stones, or someone gives you one as a gift.

If you feel you need to give the stone to someone else - do so, its your intuition telling you that they might need it more then you do. If you do not give it away, this is when one finds it has just 'disappeared'
One also often finds that when one needs the stone, it will suddenly reappear in in a completely different place.

If you feel you do not want your favourite stone near you, put it away. Listen to your intuition - your stone is there to help, and not be a crutch, which could be worse then why you needed the stone in the first place.

Sometimes crystals crack or break seemingly without reason.
They very often break when they have done their work of 'taking' negativity into themselves.
I was once selling crystals a  fair, when a young lady came up to me and asked if I had a crystal that could help with a very jealous boyfriend. We had just started speaking when he came up to the stall and asked what she was doing there. I mentioned to him that I was simply explaining crystal meanings to her, he then left. I had a crystal in my pocket of the shirt I was wearing, and suddenly felt the need to take it out. When I placed my hand in the pocket I found that the crystal had shattered into little pieces.
There are many reasons why stones break, sometimes one just has to figure it out for themselves. But in any event if the stone breaks listen to your intuition, you might have to pass the broken pieces on to others so that they might benefit from the energy of the stone.