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