While comparing Feldspar and Sapphire 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 Feldspar and Sapphire chemical formula.
Another important criteria for qualitative analysis of gemstones is Feldspar and Sapphire Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Feldspar and Sapphire Optical Properties during the identification of gemstone. Specific gravity of Feldspar is 3.95-4.03 whereas that of Sapphire is 3.95-4.03.