While comparing Selenite and Sylvite 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 Selenite and Sylvite chemical formula.
Another important criteria for qualitative analysis of gemstones is Selenite and Sylvite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Selenite and Sylvite Optical Properties during the identification of gemstone. Specific gravity of Selenite is 1.993 whereas that of Sylvite is 1.993.