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