While comparing Onyx and Demantoid physical properties, the important data you should know is its chemical composition. Since chemical formula defines the molecular structure of the crystal, most of the physical properties like color, tenacity, solubility are governed by Onyx and Demantoid chemical formula.
Another important criteria for qualitative analysis of gemstones is Onyx and Demantoid Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Onyx and Demantoid Optical Properties during the identification of gemstone. Specific gravity of Onyx is 3.77-3.95 whereas that of Demantoid is 3.77-3.95.