Why are diamonds different in appearance compared to graphite if they are both made of carbon atoms?
These are allotropes of the one element whose structures are markedly different.
Graphite is non-molecular in 2 dimensions. Graphite is an excellent lubricant inasmuch as the 2-dimensional carbon layers can slide over each other.
On the hand, the diamond structure is non-molecular in 3 dimensions. Each carbon atom is strongly bound to 4 other carbon atoms with a tetrahedral geometry, whose bonding extends indefinitely over a crystalline lattice.
This difference in structural properties gives rise to the physical difference between the carbon allotropes. Because of their non-molecularity, both graphite and diamond have exceptionally high melting points.