The problem is these are natural rock and they come filled with natural cracks which you can't see because they are so small. Over time these will grow and under certain conditions the process can accelerate. Don't confuse veins with actual cracks.

As long as the cracks are simple they can always be repaired. The tough ones are when they grow and the material starts to microchip (like a razor) which gets worse and these can be tough to repair. This is really a natural process and is how rock weathers. It's just not cool when it happens to your expensive hone in your house.