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 that makes up nacreouspearls. 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 (CIBJO), the same year commercial mining of ammolite began. It was designated the official gemstone of the City of Lethbridge in 2007.
ALSO KNOWN AS: Korite or Calcentine
CHEMISTRY: CaCO3. Variable, may include aragonite, calcite, silica, pyrite, or other minerals
REFRACTIVE INDEX: 1.52 – 1.68
HARDNESS: 4.5 – 5.5
SPECIFIC GRAVITY: 2.60 – 2.85
CLEAVAGE: None, parting present
HEAT SENSITIVE: Yes
WEARABILITY: Good to poor
SPECIAL CARE INSTRUCTIONS: See article
ENHANCEMENTS: Opticon and lacquer stabilization
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 colours
A large array of colour is displayed in ammolite, including all the spectral colours found in nature. Red and green are far more common than blue or purple due to the latter’s fragility. There are also certain hues, like crimson or violet or gold, which are derived from a combination of the primary colours, that are the rarest and in highest demand. The most valuable grades have three or more primary colours or 1–2 bright and even colours, with the lowest grades having one comparatively dull colour predominant.
The way the colours “play” (chromatic shift and rotational range)
Chromatic shift is how the colours 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 colour; the best rotate 360 degrees uncompromised, while lesser stones may exhibit highly directional colours that are only visible within a narrow rotational range, down to 90° or less. Intermediate grades have ranges of 240–180°.
Brightness of colours (iridescence)
The brightness of colours 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 that has broad, uninterrupted swathes of colour 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.
In 1981, ammolite became recognized by CIBJO, (the Coloured Stones Commission.) It is the latest of only three new gemstones introduced in the last 50 years. It is also only one of three organic gemstones, (including amber & pearl.) It has been compared to opal and has a superficial similarity to the Austrian mineral lumachelle whose iridescence is also provided by the fossil ammonite carnites floridus.
Ammonite shell is comprised primarily of aragonite, (the same mineral that makes up pearls,) with trace elements of aluminum, barium, chromium, copper, iron, magnesium, manganese, silicon, strontium, titanium and vanadium.
Unlike most other gems, whose colours comes from light refraction, the iridescent colour of ammolite comes from interference with the light that rebounds from stacked layers of thin platelets in the aragonite. Ordered thick stacks for red gems, less ordered thinner stacks for green and unordered, very thin stacks for blue ones.
The layers are .5 – .8 millimeters thick before polishing and .1 – .3 mm thick after polishing. This is sufficiently thick and durable to be manufactured into jewelry.
Ammolite is also known as aapoak, (Blackfoot for small, crawling stone,) calcentine, korite, ammonite shell or gem ammonite. Ammolite is the fossil shell of the upper cretaceous ammonites: Placenticeras meeki, Placenticeras intercalare & Baculites compressus.
Ammolite is only found in the Bearpaw formation that extends from Alberta to Saskatchewan in Canada and south to Montana in the USA. The best grade of gem quality ammolite is along high energy river systems on the eastern slopes of the Rockies in southern Alberta. Most commercial mining operations have been conducted along the banks of the St. Mary river, south of Lethbridge.
Ammolite is traditionally classified by letter grades. Jewelry is usually created from the A range grades. Normally, the higher the grade, the more brilliant the colors and the more the colors remain bright as the stone is turned in different directions.
I don’t believe an actual official grading system that is accepted by all of the various players in the “ammolite industry” currently exists – although I believe there are representatives from several groups working on one. Korite, the largest producer of ammolite, uses a letter grading system. Various sources of ammolite seem to follow this system – although they may define the grades slightly differently. The following kind of mushes together and averages out what I have found many different sources to say about how they grade ammolite:
|AA the best stones, displaying brilliant multidirectional fire with a good
representation of at least three brilliant vivid colors. Colors are sharp &
clear, with crisp distinctions.A+ at least two very bright vivid colors with little or no directional
extinction.A strong fire of one or more bright vivid colors that show from many
A- “standard” grade with good color. Colors either might not be as vivid as
B colors are not sharp and clear and may even be muddy. The stone just
C dark at most angles
Ammolite patterns are created by the size and shape of the scales, the presence or absence of fractures and/or black lines, and colors. Pattern names are used fairly loosely and, to some extent, pattern name can be an individual preference. But the more ammolite you see, the more patterns you recognize.
Dragon-skin – small scales, reminding one of the skin on a dragon or lizard
Checkerboard – larger, blockier pattern than dragon-skin
Cobblestone – like the uneven, irregular appearance of a cobblestone street
Ribbon / Banded / Striped – colors run in wide to thin bands
Wave / Ripple – water surface ripples (or their shadows) trapped in the stone
Sheet / Foil – no visible scales, although fractures and lines may be present
Moon-glow – luminescent glow, 1-2 colors, usually no fractures or other lines
Paintbrush – broad strokes of color
Lava Lamp – globules of color
Feather – includes small tendrils resembling feathers
Desert – the look of dry, parched land, with black lines a major feature.
Pin Fire – small scales emit sparks of light as the stone is turned
Flash Fire – larger scales emit vivid flashes of light as the stone is turned
There are 2 other “patterns” commonly referred to. I am putting these two separate, as I think they are more of a color variation of the above patterns.
Floral / Flower Garden – Bright red splotches, usually against a green background, resembling poppies in a field.
Stained Glass – there are some stones that just don’t fit into any of the above patterns, and that are best described as stained glass. There are others that still fit into one of the above categories, but the quality of the color and light in the stone is the same as the color and light produced by a richly colored leaded glass window. It is something that can’t really be described, but once seen, is unforgettable.
Much (not necessarily all) ammolite is micaceous – it flakes and cleaves like mica. Because of this, ammolite is stabilized. Stabilization is an accepted treatment that has no negative impact on the gem. It may enhance the gem slightly, by making it less directional.
Stabilization is done by forcing epoxy into the layers under pressure. This cements the layers together and creates a stable stone that can be used for jewelry.
Virtually all ammolite on the market has been stabilized.
Most professional lapidaries will not add any coating to the stabilized ammolite, but will just give it a final polish to show it off to its best advantage.
Natural stones may have a layer of opticon, to give a higher polish to the surface.
A clear coat is often used to protect ammonite fossils that have an outer layer of ammolite. The clear coat reflects UV rays and protects the color from fading over time and prolonged exposure to light.
Different in purpose from the above, coatings may also be used to maintain a wet look on lower quality ammolite, to improve its colour.
I have also seen a layer of automotive clear coat put on a natural stone, to give it a smooth surface. (The lapidary did not want to grind that stone down to a smooth surface, as it would have taken off some of the color layers he was trying to preserve.)
These coatings are not desirable for stones to be made into jewelry, as you do not know how the coatings will hold up to the jewelry making process. Wire jewelry is probably the most suitable method for making these stones into jewelry – but if the end of a wire or tip of a pliers touches the surface, it should not cause bubbles or flakes like a coat of bad nail polish.
When triplets (or un-capped doublets) are made, the new matrix that the ammolite is to be placed on is often made as black as possible, to intensify the colours of the ammolite. This can be done by dying the new matrix or by mixing lampblack into the epoxy.
Different effects can also be achieved by painting the matrix with a color other than black.