Question

Why is diamond hard and transparent while graphite is soft and grey even though both are made of carbon?

Answer 1

Diamond is hard because the atoms are arranged in like a honeycomb pattern which makes it hard.
Graphite is soft because the atoms are arranged in layers like the layers of onions where you can peel them off easily.
Sorry I couldn't answer your transparent and grey colour part of the question but hope this part helped you a little bit.

Answer 2

A diamond crystal is a giant molecule of carbon atoms. Each carbon atom in the diamond crystal is linked to four other carbon atoms by strong covalent bonds. The four surrounding atoms are at four vertices of a regular tetrahedron. This rigid structure of diamond makes it a very hard substance.