While comparing Mother of Pearl and Onyx 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 Mother of Pearl and Onyx chemical formula.
Another important criteria for qualitative analysis of gemstones is Mother of Pearl and Onyx Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Mother of Pearl and Onyx Optical Properties during the identification of gemstone. Specific gravity of Mother of Pearl is 2.55-2.70 whereas that of Onyx is 2.55-2.70.