While comparing Sillimanite and Cordierite Iolite 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 Cordierite Iolite chemical formula.
Another important criteria for qualitative analysis of gemstones is Sillimanite and Cordierite Iolite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Sillimanite and Cordierite Iolite Optical Properties during the identification of gemstone. Specific gravity of Sillimanite is 2.56-2.66 whereas that of Cordierite Iolite is 2.56-2.66.