While comparing Sylvite and Onyx 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 Sylvite and Onyx chemical formula.
Another important criteria for qualitative analysis of gemstones is Sylvite and Onyx Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Sylvite and Onyx Optical Properties during the identification of gemstone. Specific gravity of Sylvite is 2.55-2.70 whereas that of Onyx is 2.55-2.70.