While comparing Corundum and Sillimanite 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 Corundum and Sillimanite chemical formula.
Another important criteria for qualitative analysis of gemstones is Corundum and Sillimanite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Corundum and Sillimanite Optical Properties during the identification of gemstone. Specific gravity of Corundum is 3.20-3.26 whereas that of Sillimanite is 3.20-3.26.