Ammolite

Ammolite
Unprocessed sample of ammolite; a "dragon skin" pattern is apparent
General
Category	fossilized, mineralized Ammonite shell
Formula (repeating unit)	CaCO3aragonite polymorph, with minor amounts of calcite, pyrite, silica, and other impurities
Identification
Color	Gray to brown, can be radiant blue,with primarily red to green iridescence.
Cleavage	no true cleavage
Fracture	uneven to granular
Mohs scale hardness	3.5 - 4.5
Luster	greasy to dull
Specific gravity	usually about 2.70 (varies with mineral content)
Polish luster	vitreous
Optical properties	anomalous aggregate reaction
Refractive index	usually 1.52 - 1.68 (varies with mineral content)
Birefringence	0.135 - 0.145
Pleochroism	none
Ultraviolet fluorescence	variable

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.¹ 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, Alberta in 2007.

Ammolite is also known as aapoak (Kainah for "small, crawling stone"), 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.

The chemical composition of ammolite is variable, and aside from aragonite may include calcite, silica, pyrite, or other minerals. The shell itself may contain a number of trace elements, including: aluminium; barium; chromium; copper; iron; magnesium; manganese; strontium; titanium; and vanadium. Its crystallography is orthorhombic. Its hardness is 4.5–5.5, and its specific gravity is 2.60–2.85. The refractive index of Canadian material (as measured via sodium light, 589.3 nm) is as follows: α 1.522; β 1.672–1.673; γ 1.676–1.679; biaxial negative. Under ultraviolet light, ammolite may fluoresce a mustard yellow.