Ammolite Mineral Makeup

Ammonite shell is composed of aragonite, as are pearls. In addition trace elements of aluminum, barium, chromium, copper, iron, magnesium, manganese, silicon, strontium, titanium and vanadium creates variation.

Ammolite is relatively soft for a gem, 3.5 on the Mohs Hardness Scale. Refractive Indices ranges from 1.525 to 1.670 with little difference through the color range.

A microscopic examination of ammolite will show uniform layers (or laminae) of prismatic aragonite crystals. Depending on the type of ammolite, KZone (type 1) or BlueZone (type 2), the laminae can have a cobblestone or stained glass look with panels of color bordered with dark fracture material – or – a more continuous sheet-like  color without fracture border-liness. Type 1 material requires little stabilization while Type 2 material may need to be chemically stabilized before being made into jewellery.

Most gemstone’s colors comes from light refraction, but Ammolite’s color is iridescent,  an interference in the light rebounding from stacked layers of thin platelets in the aragonite with red being thicker and blue thinner. The layers in Triplets are usually between 0.1 mm and 0.5 mm and between 0.5 mm and 4 mm in Naturals.

Gem Forms
Ammolite is in the form of Naturals or Triplets. Naturals are free-form cabochons with a non-coated hand-finish.  Priced by carat with total thickness of no more than 4 mm.

Triplets have three distinct layers:

(1) a 2 – 4 mm base of natural bearpaw shale;

(2) Ammolite;

(3) a calibrated cap of quartz or spinel .

Iridescence

The visual property that allows us to see the incredible play of colors across Ammolite’s surface is iridescence. The shell’s multi-layered surface is composed of closely packed, tabular, crystals of aragonite (the same mineral that makes up pearls or abalone shell), with their c-axis oriented vertical to the shell surface. The crystals are bonded & united into many thin lamellae, (plates or scales.) The thickness of these lamellae is of exactly the same scale as the wavelengths of the spectral colors that make up the white light we receive from the sun. Thus, when white light enters the regularly spaced thin layers of aragonite, diffraction occurs and brilliant flashes of spectral colors are seen.
The plates of aragonite crystals vary in thickness, with blue being thinner than red in general. In addition, the trace elements are randomly arranged and interspersed with inclusions of organic material, (conchiolin.) These three factors determine the colors and intensity of the diffraction. The best ammonite shell – Ammolite – will have brilliant, vibrant iridescence, continually dancing with changing colors as the angle of the incident light changes and the stone moves with it’s owner.

Chromatic Shift
The color of ammonite depends on the angle of light and the viewer’s perspective. Sometimes subtle and sometimes spectacularly dramatic, a chromatic shift occurs. Red will shift to green, green will shift blue, etc. This is called dichromatic.
Some gem material will have the shift restricted to hues within the same primary color group. This is monochromatic. The best gem material has spectrochromatic shift, meaning the color will shift through the entire spectrum depending on light source and your angle of observation.

Rotational Range
Some material will not show strong, vibrant colors through 360° degrees of rotation. As other gems are rotated, the brightness decreases and darkens to black. This is due to the light wave diffraction being blocked by the organic inclusions in the aragonite. For an Ammolite gemstone to be Graded AA it must show a brilliant color through 360° of rotation