Why diamond is hard compared with graphite?

Graphite is a

The index of refraction of diamond is 2.0, velocity of light in diamond in cm per second is approximately :

Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green? Why? Answer

If we burn graphite

Why do veins have thin walls as compared to arteries ?