While comparing Howlite and Zircon 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 Howlite and Zircon chemical formula.
Another important criteria for qualitative analysis of gemstones is Howlite and Zircon Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Howlite and Zircon Optical Properties during the identification of gemstone. Specific gravity of Howlite is 3.93-4.73 whereas that of Zircon is 3.93-4.73.