While comparing Sillimanite and Polyhalite 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 Polyhalite chemical formula.
Another important criteria for qualitative analysis of gemstones is Sillimanite and Polyhalite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Sillimanite and Polyhalite Optical Properties during the identification of gemstone. Specific gravity of Sillimanite is 2.78 whereas that of Polyhalite is 2.78.