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