While comparing Emerald and Opal 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 Emerald and Opal chemical formula.
Another important criteria for qualitative analysis of gemstones is Emerald and Opal Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Emerald and Opal Optical Properties during the identification of gemstone. Specific gravity of Emerald is 1.98-2.25 whereas that of Opal is 1.98-2.25.