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