Diamond is the hardest natural substance on Earth.
It is made 100% of carbon, just like graphite. Nevertheless, pencil's mines may seem very fragile in comparison to diamonds, well actually the difference between your pencil's mine and diamonds is the type of bond between their carbon atoms. This encourages us to believe that strength depends on the nature of the bonds between the atoms, this is why, to us, diamonds symbolise unbreakable bonds between people.
Regarded as God's tears by ancient Romans and Greeks, humans have ever since been fascinated by these precious stones.