Biotite and Feldspar optical properties play a significant role while distinguishing these gemstones from each other. These properties are influenced by the Biotite and Feldspar Physical Properties like crystallographic symmetry, pleochroism,etc. in the crystal. Some of the optical properties are refractive index, luster, optic character, pleochroism etc. Prominent among these is Biotite and Feldspar optic character. You might have noticed your gemologist rotating the gemstone while testing it. Rotating a gemstone along its optic axis helps in determining its optic character.
Similar to their colors, Biotite and Feldspar transparency can also be used as criteria for their primary identification. Although transparency cannot be the only criteria for gemstone identification, it can efficiently used in gemstone grading. Talking about their transparency, Biotite is while Biotite is in appearance.