What type of crystalline solid is a diamond?

Diamond is a network covalent solid with no molecular boundaries.

Each carbon atom in diamond is covalently bound to 4 other carbon atoms in a tetrahedral array. Because there are NO molecular boundaries, the physical properties of diamond, melting point/boiling point, are so high as to be almost unmeasurable. To break up the solid structure, enough energy has to be pumped in to break STRONG ##C-C## bonds that are covalently bound in an infinite array.