Diamonds are hard in the same way glass is hard. You can't scratch it with any other material but it's very fragile because it's a carbonite crystalline structure.

The outermost shell of each carbon atom has four electrons. In diamond, these electrons are shared with four other carbon atoms to form very strong chemical bonds resulting in an extremely rigid tetrahedral crystal.

It is precisely because it is carbon formed in such a structure that it is hard. Take away the carbon or the structure (pretty sure it's not easy to take away one without the other), diamond loses its incredible hardness. As far as I know, nothing can be compared to this monster.

Hardness is understood here as a material's ability to withstand friction, essentially abrasion resistance or in simpler words, scratching. It is not the same as "toughness" which is the ability of a material to absorb energy and plastically deform without fracturing.

Diamond's crystalline structure is its Achilles's heel when it comes to toughness.