Why do the minerals graphite and diamond look different even though they’re made from the same element, carbon (c)?

Since they are both made of carbon their chemical properties are the same.

Most physical properties of a solid like hardness, melting point, conductivity of heat/electricity, etc. largely depend on it's crystal structure.

Graphite and diamond have very different structures.

Graphite has a layered sheet-like structure with each layer consisting of hexagonal rings, this is because all carbon atoms are sp2 hybridised.

Diamond has a tetrahedral structure since all carbon atoms are sp3 hybridised.

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