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