While comparing Sillimanite and Feldspar 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 Feldspar chemical formula.
Another important criteria for qualitative analysis of gemstones is Sillimanite and Feldspar Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Sillimanite and Feldspar Optical Properties during the identification of gemstone. Specific gravity of Sillimanite is 2.56-2.75 whereas that of Feldspar is 2.56-2.75.