While comparing Biotite and Gahnite 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 Biotite and Gahnite chemical formula.
Another important criteria for qualitative analysis of gemstones is Biotite and Gahnite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Biotite and Gahnite Optical Properties during the identification of gemstone. Specific gravity of Biotite is 4.00-4.62 whereas that of Gahnite is 4.00-4.62.