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