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