Peridot (magnesium iron silicate)
Color: Yellow-green, apple-green, olive-green to brownish.
Moh’s hardness: 6.5-7
Refractive index: 1.654-1.69
Critical angle: 37°
Specific gravity: 3.27-3.37
Fracture: Brittle, small conchoidal.
Dispersion: Moderately high (.02)
Heat sensitivity: Low to medium
Pleochroism: Dichroic – very weak (colorless or light green to deeper yellow-green or olive-green).
Birefringence (double refraction): Medium (.036)
Crystal structure: Orthorhombic
Treatments: Peridot may be oiled or "opticoned" to reduce the visibility of flaws. These are not accepted treatments.
Polishing: Alumina on tin or tin/lead. If polishing becomes problematic, add a small amount of vinegar to the water drip or use 50,000 diamond on a tin/lead lap. Peridot has a reputation for being difficult to polish. When this occurs, it is usually only one or two facets that cause the difficulty.
Comments: It is common for peridot to contain small black inclusions and some silk. The most problematic inclusions in peridot are called “lily pads.” Lily pads are difficult to see and cause many of the problems in faceting peridot. They are flat round inclusions that act like cleavage planes when polishing. If the lily pads do not come to the surface of the stone, and are parallel to the table, they may not be visible to the unaided eye in the finished stone.