While comparing Feldspar and Goethite 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 Goethite chemical formula.
Another important criteria for qualitative analysis of gemstones is Feldspar and Goethite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Feldspar and Goethite Optical Properties during the identification of gemstone. Specific gravity of Feldspar is 4.27-4.29 whereas that of Goethite is 4.27-4.29.