About 200 years ago it was discovered that diamond, like graphite, was made entirely of carbon. One brilliantly reflective, the other black; one hard, the other soft. How was it possible that two things with properties so contrasted could be made of the same thing? With this discovery came an extraordinary insight: what mattered was not the element itself, for the single carbon atom in isolation had no particular properties. What mattered was the bond structure.
So what does this mean? A chemical bond is simply a probability of how much time electrons from one atom spend hanging around in the space of another. In the case of the diamond the carbon atoms are strongly bonded to each of their four closest neighbours giving it the property of hardness. And so closely engaged are these atoms that when light energy enters it is not welcomed and it bounces around simply leaving the system giving it its reflective sparkle. In contrast, in graphite the carbon atoms are not all tightly bonded but rather some of them associate with one another in more fluid nature giving it softness and the ability to accept or absorb light energy to make it its own.