While comparing Feldspar and Willemite 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 Willemite chemical formula.
Another important criteria for qualitative analysis of gemstones is Feldspar and Willemite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Feldspar and Willemite Optical Properties during the identification of gemstone. Specific gravity of Feldspar is 3.89-4.18 whereas that of Willemite is 3.89-4.18.