While comparing Amethyst and Biotite 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 Amethyst and Biotite chemical formula.
Another important criteria for qualitative analysis of gemstones is Amethyst and Biotite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Amethyst and Biotite Optical Properties during the identification of gemstone. Specific gravity of Amethyst is 2.7-3.3 whereas that of Biotite is 2.7-3.3.