Silicon Carbide (industrially known as Carborundum) is extremely rare in nature, but has been produced in quantity since the 1890's.  Its hardness has found many uses in industry;  diamond jewelry has been produced by this compound that have nearly fooled experts.  The basic structure is a tetrahedron, similar to a molecule of methane, which contains a carbon surrounded by 4 hydrogen atoms.  Substitute 4 silicon atoms for the hydrogen and you have a molecule of silicon carbide.  Although the silicon is held in the tetrahedral structure by covalent bonds with the carbon, their interaction is such that a slight ionic bond is created as well; the carbon becoming slightly negative and the silicons slightly positive.  The polar qualities, or intermolecular forces, similar to what is found in water molecules, cause the various layers of tetrahedrons to adhere strongly to each other.  Overcoming these forces takes a substantial quantity of heat, thus the high melting point of SiC.

Lithium Fluoride, although strongly ionically bonded, has no polar qualities, so its melting point is far less.

See link for structure diagrams: