page_title: Ammolite
section_title: Gemstone quality
hierarchical_section_title: Ammolite / Gemstone quality
caption_attribution_description: Ammolite (5.5 cm across at its widest) - diagenetically altered, iridescent Placenticeras ammonite shell material (nacreous aragonite, CaCO3) from the Upper Cretaceous Bearpaw Formation of the St. Mary River Valley, Alberta, Canada. Ammolite is biogenic gem material from Alberta, Canada. It has stunningly intense, iridescent rainbow colors. Ammolite is fossil shell material from Placenticeras ammonites. Ammonites are an extinct group of swimming squid-like organisms with planispirally coiled shells (the chambered nautilus in modern oceans is a distant relative of ammonites, but has a similar body plan). Ammonite shells were originally nacreous aragonite (“mother of pearl”) (CaCO3). Geologic studies have shown that ammolite gem material formed from slight diagenetic alteration of the original ammonite nacreous aragonite shell. Diagenesis has significantly intensified and brightened the play of colors. Ammolite is mined, polished, and treated by resin- or epoxy-impregnation to stabilize it. Very rarely, complete specimens of Placenticeras ammonite shells preserved in ammolite are recovered - such specimens are exceedingly valuable (for example, see figure 2 of Mychaluk et al., 2001). Name &amp; classification: Placenticeras meeki or Placenticeras intercalare (Animalia, Mollusca, Cephalopoda, Ammonoidea, Ammonitina) Stratigraphy &amp; age: Bearpaw Formation, Campanian Stage, upper Upper Cretaceous, ~70-75 Ma. Locality: mine in the St. Mary River Valley west or northwest of Welling and south-southwest of Lethbridge, southern Alberta, southwestern Canada. Reference cited: Mychaluk, K.A., A.A. Levinson &amp; R.L. Hall. 2001. Ammolite: iridescent fossilized ammonite from southern Alberta, Canada. Gems &amp; Gemology 37(1): 4-25.
context_page_description: Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell. It is one of few biogenic gemstones; others include amber and pearl. In 1981, ammolite was given official gemstone status by the World Jewellery Confederation, the same year commercial mining of ammolite began. It was designated the official gemstone of the City of Lethbridge, Alberta in 2007.
  Ammolite is also known as aapoak, gem ammonite, calcentine, and Korite. The latter is a trade name given to the gemstone by the Alberta-based mining company Korite. Marcel Charbonneau and his business partner Mike Berisoff were the first to create commercial doublets of the gem in 1967. They went on to form Ammolite Minerals Ltd.
context_section_description: 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.
caption: Placenticeras ammolite specimen, from  Bearpaw Formation, Campanian Stage, upper Upper Cretaceous, ~70-75 Ma.
exif: {"Image ImageDescription": "", "Image Make": "Canon", "Image Model": "Canon PowerShot D10", "Image Orientation": "Horizontal (normal)", "Image XResolution": "180", "Image YResolution": "180", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop CS2 Macintosh", "Image DateTime": "2014:09:14 21:03:11", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "272", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "1194", "Thumbnail JPEGInterchangeFormatLength": "6363", "EXIF ExposureTime": "1/100", "EXIF FNumber": "49/10", "EXIF ISOSpeedRatings": "320", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2012:09:17 15:44:27", "EXIF DateTimeDigitized": "2012:09:17 15:44:27", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "3", "EXIF ShutterSpeedValue": "213/32", "EXIF ApertureValue": "147/32", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "147/32", "EXIF MeteringMode": "Pattern", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "93/5", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "2248", "EXIF ExifImageLength": "1656", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "Interoperability RelatedImageWidth": "4000", "Interoperability RelatedImageLength": "3000", "EXIF InteroperabilityOffset": "1044", "EXIF FocalPlaneXResolution": "4000000/243", "EXIF FocalPlaneYResolution": "1500000/91", "EXIF FocalPlaneResolutionUnit": "2", "EXIF SensingMethod": "One-chip color area", "EXIF FileSource": "Digital Camera", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF DigitalZoomRatio": "1", "EXIF SceneCaptureType": "Standard"}