different crystal structures
Sometimes two different minerals have the same chemical composition. But they are different minerals because they have different crystal structures. Diamonds are very valuable as gemstones because they are very pretty and very hard. Graphite is the lead in pencils. It's not hard at all! Amazingly, both are made just of carbon. Compare the diamond with the pencil lead Figure below . Why are they so different? The carbon atoms in graphite bond to form layers. The bonds between each layer are weak. The carbon sheets can just slip past each other. The carbon atoms in diamonds bond together in all three directions. This strong network makes diamonds very hard.