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