While comparing Obsidian and Geodes 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 Geodes chemical formula.
Another important criteria for qualitative analysis of gemstones is Obsidian and Geodes Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Obsidian and Geodes Optical Properties during the identification of gemstone. Specific gravity of Obsidian is Not Available whereas that of Geodes is Not Available.