Giant Covalent Substances
Atoms of some elements can form several covalent bonds. These atons can join together in giant covalent structures (sometimes called 'macromolecules). Ever atom in the structure is joined to several other atoms by strong covalent bonds. It takes an enormous amount of energy to break down the lattice and so these substances have very high melting points.
Diamond (a form of carbon) and silica (silicon dioxide) have regular three-dimensional giant structures and so they are very hard and transparent.
Graphite is a form of carbon in which the atoms join in flat two-dimensional layers. There are only weak forces between the layers and so they slide over each othe, making graphite slippery and grey.