Which property can't be determined if the gem is mounted?

• A. Color
• B. Refractive Index
• C. Specific Gravity
• D. Fluorescence

Answer: (C)

Specific gravity relies on weighing the stone in air and in water to determine its density, and this measurement must reflect only the gem’s mass and volume. When the gem is set in a mounting, the setting contributes weight and displaces water too, so you can’t isolate the stone’s true mass and volume. The result is an SG that’s contaminated by the mount, making an accurate determination impossible while the gem remains mounted. If you remove the gem from the setting, you can measure its SG reliably.

Color can still be observed through or around the setting (though the background can influence perception). Fluorescence is detectable under UV light and isn’t inherently blocked by mounting. Refractive index, however, requires the stone to be placed on a refractometer prism with immersion fluid, so the mounting typically prevents a proper RI measurement until the gem is unmounted.