You don't need to know much more about Diamond now than you did at GCSE.
You should still be able to explain its lack of conductivity in terms of a lack of delocalised electrons, and its use in drills and saws in terms of the hardness caused by the strength of its four Covalent bonds.
You should now be able to add the idea that the four tetrahedrally arranged bonds use up all the outer (valence) electrons, meaning that there are none left over to delocalise and charge charge in an electric current.
You should now be able to explain the melting point being almost so high as a result of the strength of the covalent bonds which must break for the structure to melt - this require a great deal of energy and hence a very high temperature.
You should be able to suggest that it is the highly ordered "Giant Covalent Structure" that allows light to pass through diamonds so easily, making it transparent while Graphite's much less ordered lattice is opaque.
You should also know that Silicon has precisely the same structure
That is all!