The quality of gem ammolite is communicated via a letter grade system, from most desirable to least desirable: AA; A+; A; and A-.
However, this system is not yet standardized and some vendors may use their own systems.
The grade and therefore the value of an ammolite gemstone is determined by the following criteria:
The number of primary colors 
A large array of color is displayed in ammolite, including all the spectral colors found in nature.
Red and green are far more common than blue or purple due to the latter's fragility (see properties).
There are also certain hues, like crimson or violet or gold, which are derived from a combination of the primary colors, that are the rarest and in highest demand.
The most valuable grades have three or more primary colors or 1–2 bright and even colors, with the lowest grades having one comparatively dull color predominant.

The way the colors "play" (chromatic shift and rotational range) 
Chromatic shift is how the colors vary with the angle of viewing and the angle of light striking the gemstone.
In higher grades this variation is almost prismatic in its scope, while lower grades show very little variation.
Rotational range is how far the specimen can be turned while maintaining its play of color; the best rotate 360 degrees uncompromised, while lesser stones may exhibit highly directional colors that are only visible within a narrow rotational range, down to 90° or less.
Intermediate grades have ranges of 240–180°.

Brightness of colors (iridescence) 
The brightness of colors and their iridescence is essentially dependent on how well-preserved the nacreous shell is, and how fine and orderly the layers of aragonite are.
The quality of the polish is also a factor.
The "dragon skin" cracking usually hinders its value; the most prized ammolite is the sheet type (see formation) that has broad, uninterrupted swathes of color similar to the "broad flash" category of opal.
The matrix is not visible in finer grades, and there should be no foreign minerals breaking up or diminishing the iridescence.

The thickness of the ammolite layer is also an important factor: after polishing, the ammolite is only 0.1–0.3 millimeters thick.
The rarest and most valuable are thick enough to stand alone, with only a thin portion of its original matrix (not exceeding 1.5 mm); but the vast majority require some sort of supportive backing.
Other treatments are also commonly undertaken; all other factors being equal, the less treatment an ammolite gem has received, the more valuable it is.
Calibrated stones—that is, stones fashioned into standard dimensions that will fit most jewelry settings—may also command a higher price.
The quality of gem ammolite is communicated via a letter grade system, from most desirable to least desirable: AA; A+; A; and A-. However, this system is not yet standardized and some vendors may use their own systems. The grade and therefore the value of an ammolite gemstone is determined by the following criteria:
The number of primary colors 
A large array of color is displayed in ammolite, including all the spectral colors found in nature. Red and green are far more common than blue or purple due to the latter's fragility (see properties). There are also certain hues, like crimson or violet or gold, which are derived from a combination of the primary colors, that are the rarest and in highest demand. The most valuable grades have three or more primary colors or 1–2 bright and even colors, with the lowest grades having one comparatively dull color predominant.
The way the colors "play" (chromatic shift and rotational range) 
Chromatic shift is how the colors vary with the angle of viewing and the angle of light striking the gemstone. In higher grades this variation is almost prismatic in its scope, while lower grades show very little variation. Rotational range is how far the specimen can be turned while maintaining its play of color; the best rotate 360 degrees uncompromised, while lesser stones may exhibit highly directional colors that are only visible within a narrow rotational range, down to 90° or less. Intermediate grades have ranges of 240–180°.
Brightness of colors (iridescence) 
The brightness of colors and their iridescence is essentially dependent on how well-preserved the nacreous shell is, and how fine and orderly the layers of aragonite are. The quality of the polish is also a factor. The "dragon skin" cracking usually hinders its value; the most prized ammolite is the sheet type (see formation) that has broad, uninterrupted swathes of color similar to the "broad flash" category of opal. The matrix is not visible in finer grades, and there should be no foreign minerals breaking up or diminishing the iridescence.
The thickness of the ammolite layer is also an important factor: after polishing, the ammolite is only 0.1–0.3 millimeters thick. The rarest and most valuable are thick enough to stand alone, with only a thin portion of its original matrix (not exceeding 1.5 mm); but the vast majority require some sort of supportive backing. Other treatments are also commonly undertaken; all other factors being equal, the less treatment an ammolite gem has received, the more valuable it is. Calibrated stones—that is, stones fashioned into standard dimensions that will fit most jewelry settings—may also command a higher price.
Ammolite