While comparing Polyhalite and Brucite 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 Polyhalite and Brucite chemical formula.
Another important criteria for qualitative analysis of gemstones is Polyhalite and Brucite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Polyhalite and Brucite Optical Properties during the identification of gemstone. Specific gravity of Polyhalite is 2.39 whereas that of Brucite is 2.39.