How to Grade the Color of an Amethyst

Choose a workspace with white or neutral walls and without colored curtains, since light reflecting off colored surfaces skews observation.

Work near a window to get natural light, since this light source most accurately represents a stone's color. Incandescent lighting is also recommended. Examining an amethyst in both light sources allows for a more accurate color analysis. Avoid fluorescent light, as it artificially tints stones and skews color grades.

Clean the gem, holding the stone with tweezers over a white surface and gently rubbing it with a gem cleaning cloth to remove fingerprints, dust and dirt and to provide a soft polish.

Evaluate the gem's hue. The hue is the color or tint, categorized as either violet, red, orange, yellow, green, blue or purple. Note the stone's primary hue first. If other colors are present, use the words "slightly" and "strong" to describe the amount of color. For example, if an amethyst is mostly purple with a hint of red, it is described as "slightly reddish purple". A more pronounced red would be "strong reddish purple". If the two colors are evenly blended, it is "red-purple." When grading, gemologists use a standard method of abbreviation. The primary color is capitalized and all other letters are in lowercase. Use "sl" for slightly and "st" for strong. For example, slrP is slightly reddish purple and strP is strong reddish purple.

Assess the amethyst's tone. Tone refers to a gem's relative lightness or darkness on a scale from 0 (colorless) to 10 (black). A high-grade amethyst scores a 7 on tone. When grading tone, look straight down on top of the gem, since viewing at an angle affects the brightness.

Evaluate the gem's saturation -- the intensity of its hue. Saturation is graded on six levels, ranging from brownish to vivid. Saturation is difficult to distinguish from tone, but a stone's saturation makes the gem appear brighter. However, saturation is judged according to the stone's species. For instance, a high-grade amethyst usually scores a 5, or medium, in saturation. To aid in distinguishing saturation from tone, many gemologists use synthetic high-grade gems to compare samples with when grading. When writing your final color description, list the hue first, then the tone, and then saturation. For example, an amethyst may be described as strP, 6/4, meaning "strong reddish purple," tone 6, and saturation level 4.