While comparing Boulder Opal and Sellaite 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 Boulder Opal and Sellaite chemical formula.
Another important criteria for qualitative analysis of gemstones is Boulder Opal and Sellaite Specific gravity. Specific gravity is the relative density of a gemstone compared with respect to density of water. Gemologists use Boulder Opal and Sellaite Optical Properties during the identification of gemstone. Specific gravity of Boulder Opal is 3.14-3.16 whereas that of Sellaite is 3.14-3.16.