The graphite inside a pencil and a diamond in a ring are chemically identical — both are made of nothing but carbon atoms, with no other element involved. One is soft enough to leave a mark by rubbing against paper. The other is the hardest naturally occurring substance known. Neither fact has anything to do with the atoms themselves; it comes entirely from how those identical atoms are arranged and bonded to each other, a difference chemistry calls allotropy.
Two structures, two completely different sets of bonds
In diamond, every carbon atom bonds covalently to four neighbouring carbon atoms, arranged in a rigid three-dimensional lattice called a tetrahedral structure, with strong bonds extending equally in all directions throughout the entire crystal. That structure is what makes diamond exceptionally hard: breaking it apart means breaking a dense, uniform network of strong covalent bonds running through the whole material, not just a weak point somewhere. In graphite, each carbon atom bonds to only three neighbours, forming flat, hexagonal sheets — strongly bonded within each sheet, but held to the sheets above and below only by comparatively weak forces between layers. That's why graphite is soft and slippery: whole sheets slide past each other easily, leaving a smear of carbon behind, which is literally what happens when a pencil "writes."
Same atom, wildly different everything
Allotropes — different structural forms of the same element — don't just differ in hardness. Diamond is a poor conductor of electricity because all four of each carbon atom's bonding electrons are locked into fixed covalent bonds, with none free to carry a current. Graphite, by contrast, conducts electricity reasonably well along its sheets, because each carbon atom's fourth electron isn't tied down in the same rigid way and can move relatively freely across the layer. Two more allotropes of pure carbon, discovered far more recently — spherical "buckyball" molecules and single-layer graphene sheets — extend the same basic lesson even further: identical atoms, radically different structures, radically different properties, from electrical conductivity to strength to how the material even looks.
What we're still unsure about
The structural explanation for diamond's hardness and graphite's softness is settled, well-modelled solid-state chemistry, not a live scientific dispute. What remains an active area of research is engineering new carbon allotropes and predicting their properties before they're actually synthesised — graphene's discovery in 2004 came as a genuine surprise to much of the field, since a stable, isolated single-atom-thick sheet had been widely assumed to be thermodynamically impossible at any usable temperature. How many more genuinely novel, stable carbon structures remain undiscovered, and what unexpected properties they might have, is still an open and actively pursued question in materials chemistry.
This sits inside Solid-State Chemistry & Crystal Structures, one of seven topics in Inorganic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.