While comparing Obsidian and Alexandrite 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 Alexandrite chemical formula.
Another important criteria for qualitative analysis of gemstones is Obsidian and Alexandrite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Obsidian and Alexandrite Optical Properties during the identification of gemstone. Specific gravity of Obsidian is 3.69-3.81 whereas that of Alexandrite is 3.69-3.81.