Why is diamond so hard even through it is basically graphite, a substance used in pencil lead?

The hardness of minerals is measured on a scale of 1- 10, which was devised in 1822 by an Austrian named Friedrich Mohs. Number 1 on Mohs' scale is talc, an extremely soft mineral. Number 10 is diamond, the hardest natural substance. When cut and polished, diamond is prized in jewellery, but the only thing that can be used to cut and polish diamond is diamond itself. Diamond, which is about 40 times as hard as talc, is made of pure carbon. Chemically, it is exactly the same as graphite, the substance used to make pencil lead. Graphite has a hardness of between 1 and 2 on the Mohs' scale. The difference between hard carbon and soft graphite is caused by the arrangement of the carbon atoms. In graphite, the atoms are linked in flat planes which readily slide over each other. In diamond, which is formed in great heat and under intense, pressure, the atoms are bound in a rigid structure.