Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course

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AMMOLITE

Ammolite is the accepted trade and variety name for iridescent fossilized ammonite shell from a single deposit in southern Alberta, Canada. Only two species of the many types of these ancient snail-like creatures yielded this gem quality material when fossilized.

[A high grade ammolite gemstone. A close up of the iridescent surface of an ammolite gem]

Other ammonite species subject to different fossilization processes have left casts, impressions and petrified remains with and without pyritization in many other places around the world.

[Fossil Ammonites (not Ammolite): petrified animal showing the interior chambers, pyritized ammonite fossil showing some of exterior shell detail]

A relative newcomer to the gem scene, ammolite began to enter the world market only during the 1960's. The iridescence is created by interference between light waves travelling through layers of thin, tablet-like aragonite crystals. The thickness of the individual crystals determines the color seen, with thicker ones giving red and the thinnest ones creating purple. These structural differences help explain why the red material tends to be tougher and more common than the blue or purple. Consequently the reds cost less than blues, all else being equal.

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[Red colors mean thick layers of aragonite, blue and purple indicate thin ones]

The iridescent layer of these fossils is generally very thin, no more than 8 mm before polishing and as thin as .1 to 3 mm afterward. For this reason and because of its low hardness (3.5) most ammolite is either stabilized by vacuum assited impregnation with a plastic resin, coated with lacquer, or made into assembled stones such as doublets or triplets. With such treatment, ammolite is suitable for jewelry use.

[Ammolite pendants in gold]

In order of rarity the colors are: red, gold, green, blue-green, blue, violet and purple. Patterning varies, with some pieces showing color sections in big blocks, others with finely veined netting and still other pieces showing a single solid color.

[Ammolite gems showing: red/orange color (more common) and blue green (rarer), the multicolored carved piece shows the full range of colors as well as characteritic netting and veining]

Ammolite is a spectacular, under-used gem material with tremendous potential for greater use by designers as long as the settings are protective or assembled or coated pieces are used. Prices are relatively high for a cabochon material, but with its vivid colors and distinctive patterns, ammolite holds up very well in an aesthetic comparison with black opal which is at least an order of magnitude more expensive.

Value

The value of a piece of ammolite is determined by several different factors: the nature and intensity of the colors, the presence or absence of dead spots or directionality in the iridescence and the degree of fracturing. The piece below shows the desirable characteristic of full iridescence, regardless of viewing angle in addition to rare blue color.

Added to this, is a premium which is accorded to solid pieces (non-assembled) which have received no treatment beyond basic resin impregnation. Pieces without stabilization are a possibility for the collector market, but use of such pieces for jewelry is way too risky. Although little has been written of a specific nature about ammolite prices, it is reasonable to expect premium prices for large sizes, rare colors, attractive patterns and freedom from blemishes.


Gemological Properties:

Makeup: fossilized mineralized ammonite shell, usually aragonite: CaC03
Hardness: 3.5
Crystal System: Orthorhombic
Luster: vitreous to resinous
Toughness: red is relatively tough, blue and purple are brittle
Density: usually about 2.76-2.84
RI: usually 1.52-1.67
Birefringence: 0.135 - 0.145
Cleavage: usually none
Stones Currently Available:

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Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course

 -- Mail to barbara@acstones.com