While comparing Obsidian and Fayalite 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 Obsidian and Fayalite chemical formula.
Another important criteria for qualitative analysis of gemstones is Obsidian and Fayalite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Obsidian and Fayalite Optical Properties during the identification of gemstone. Specific gravity of Obsidian is 4.392 whereas that of Fayalite is 4.392.