True, I believe that everything in nature has something in store for us, if we can tap it nothing like it.
Personally I have experienced that when I get drawn very strongly to a crystal then at that point of time it is the most apt one for me and I pick it up, also the observation is that after using it for sometime then many a times I don't touch it for months. So it comes when it is most required for your situation and when the purpose is met it goes away. The journey with them is always very beautiful for me.