Clarkeite is a uranium oxide mineral of composition (Na,Ca,Pb)₂(UO₂)₂(O,OH)₃ or (Na,Ca,Pb)(UO₂)O(OH)·0-1H₂O. Cinnabar is generally found in a massive, granular or earthy form and is bright scarlet to brick-red in color. It occasionally occurs in crystals with a non-metallic adamantine luster. Cinnabar has a rhombohedral bravais lattice, and belongs to the hexagonal crystal system, trigonal division. Its crystals grow usually in a massive habit, though they are sometimes twinned. The twinning in cinnabar is distinctive and forms a penetration twin that is ridged with six ridges surrounding the point of a pyramid. It could be thought of as two scalahedral crystals grown together with one crystal going the opposite way of the other crystal. The hardness of cinnabar is 2–2.5, and its specific gravity 8.1.

Cinnabar resembles quartz in its symmetry and certain of its optical characteristics. Like quartz, it exhibits birefringence. It has the highest refractive power of any mineral. Its mean index for sodium light is 3.08, whereas the index for diamond is 2.42 and that for gallium(III) arsenide (GaAs) is 3.93.