Salt, magnesium oxide (magnesia), calcium chloride, alumina, silica, Silicon carbide, zirconium oxide, tungsten carbide, boron carbide.
Characterized by having high melting / boiling point, brittleness, high hardness. Crystalline solids.
Any plastics will do: Polyethylene, polypropylene, ABS
Any other organic compounds: ethanol, ethylene, wax etc.
Solids, liquids and gases with lower melting/boiling points, typically softer and tougher. Thermal and electrical insulators
Try some metals: Iron, Zinc, Aluminum, Titanium, Copper. Thermal and electrical conductors, malleable and ductile