Goethite and Sillimanite optical properties play a significant role while distinguishing these gemstones from each other. These properties are influenced by the Goethite and Sillimanite 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 Goethite and Sillimanite 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, Goethite and Sillimanite 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, Goethite is Gemstones (2009) while Goethite is Gemstones (2009) in appearance.