While comparing Hercynite and Crystal 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 Hercynite and Crystal Opal chemical formula.
Another important criteria for qualitative analysis of gemstones is Hercynite and Crystal Opal Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Hercynite and Crystal Opal Optical Properties during the identification of gemstone. Specific gravity of Hercynite is 1.97-2.22 whereas that of Crystal Opal is 1.97-2.22.