Grandidierite and Sillimanite optical properties play a significant role while distinguishing these gemstones from each other. These properties are influenced by the Grandidierite 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 Grandidierite 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, Grandidierite 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, Grandidierite is Gemmological Tables (2004) More from other references while Grandidierite is Gemmological Tables (2004) More from other references in appearance.