While comparing Black Opal and Alexandrite 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 Black Opal and Alexandrite chemical formula.
Another important criteria for qualitative analysis of gemstones is Black Opal and Alexandrite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Black Opal and Alexandrite Optical Properties during the identification of gemstone. Specific gravity of Black Opal is 3.69-3.81 whereas that of Alexandrite is 3.69-3.81.