While comparing Kainite and Apatite 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 Kainite and Apatite chemical formula.
Another important criteria for qualitative analysis of gemstones is Kainite and Apatite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Kainite and Apatite Optical Properties during the identification of gemstone. Specific gravity of Kainite is 3.16-3.23 whereas that of Apatite is 3.16-3.23.