Colored Stone Essentials and Colored Stone Grading
What this domain tests
Colored Stone Essentials and Colored Stone Grading tests how GIA describes and grades colored stones: hue, tone, saturation, clarity type, cut, phenomena, and treatment disclosure. It also tests major gem groups and market terminology, but not diamond-style cut grades.
Core rules
- Color is described by hue, tone, and saturation. Hue names the dominant color plus modifiers; the dominant hue is the noun, so violetish blue is blue with a violet modifier, while reddish orange is orange with a red modifier. Tone runs very light to very dark; saturation runs grayish or brownish through slightly, moderately, strongly, to vivid.
- Clarity types reflect what is normal for the species. Type I stones are usually eye-clean (aquamarine, blue topaz, citrine); Type II usually contain inclusions (ruby, sapphire, tourmaline, garnet, spinel); Type III are almost always included (emerald). Clarity is judged at about 6 inches (15 cm) under diffused light, not at 10× like diamond.
- Cut is judged by face-up appearance, color, weight retention, and finish. Windowing (a see-through area), extinction (dark areas), and poor symmetry lower appeal; colored stones are not assigned Excellent-to-Poor cut grades.
- Phenomena include asterism, chatoyancy, play-of-color, adularescence, labradorescence, color change, and iridescence.
- Treatments must be disclosed: heat on ruby and sapphire, irradiation on blue topaz, fracture filling on emerald, diffusion on sapphire, dyeing on jade and turquoise, impregnation, and bleaching.
Worked example
A cushion-cut sapphire is described as violetish blue, medium dark tone, strong saturation, Type II, eye-clean. Blue is the dominant hue; violet is the modifier. Medium dark tone means it is darker than medium but not dark. Strong saturation means the color is strong but not quite vivid. Type II means inclusions are expected, yet this stone is eye-clean at the stated viewing distance. Heat treatment is common for sapphire and must be disclosed; it does not make the stone synthetic.
Traps
Do not reverse hue modifiers, confuse tone with saturation, or apply diamond clarity or cut grades to colored stones. Do not treat AAA as a GIA grade. Do not ignore treatment status or assume a treated stone is synthetic. Do not judge clarity with a 10× loupe; use the colored-stone eye-clean standard.