Amblygonite Value, Price, and Jewelry Information

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Oval-cut amblygonite, 7.80 cts, 15.5 x 10.8 x 7.5 mm, light yellow, Myanmar. ? ARK Rare Gems. Used with permission.

Amblygonite

Amblygonite gems are usually pale straw yellow. Although they are too soft and cleavable to make good ring stones, collectors prize them if they show darker colors. Large faceted stones are extremely rare.

Amblygonite Information

Data	Value
Name	Amblygonite
Formula	(Li, Na) Al (PO₄)(F, OH). Usually Li greatly exceeds Na.
Etymology	Amblygonite comes from the Greek amblus for “blunt” and gonia for “angle,” alluding to the shapes of its crystals. Montebrasite is named after its type locality, the French town of Montebras.
Occurrence	Granite pegmatites.
Inclusions	Commonly, veil-type inclusions, usually clouds in parallel bands.
Colors	Colorless, white, grayish white, yellow, pinkish, tan, greenish, bluish, lilac.
Fracture	Uneven
Hardness	5.5-6
Cleavage	Perfect 1 direction, good 1 direction
Crystallography	Triclinic; crystals equant to short prismatic, rough faces. Twinning common. Usually in cleavable masses.
Refractive Index	1.578-1.633
Birefringence	0.020-0.022
Dispersion	0.014-0.015
Luminescence	Pale blue in SW (Keystone, South Dakota). Weak orange or bright green in LW, or pale brown in LW (Pala, California).
Absorption Spectrum	Not diagnostic
Pleochroism	None
Optics	See "Identifying Characteristics" below.
Optic Sign	Biaxial -
Luminescence Present	Yes
Luminescence Type	Fluorescent, UV-Long, UV-Short
Luster	Vitreous to greasy; pearly on cleavages.
Specific Gravity	Approximately 2.98-3.11. Amblygonite = 3.11; montebrasite = 2.98. (Natromontebrasite* = 3.04-3.1).
Transparency	Translucent to transparent

Cut amblygonites,?Minas Gerais, Brazil. Gem cutting by Afonso Marques. Photo by Eurico Zimbres. Licensed under CC By-SA 3.0.

Comments

Amblygonite and montebrasite form a mineral series. Fluorine-dominant (F) amblygonites are much rarer than hydroxyl-dominant (OH) montebrasites. In addition, amblygonite has a biaxial (-) optic sign, while montebrasite has a biaxial (+) optic sign. Many gems labelled as amblygonites in collections may merit reexamination.

Most yellow gems in this series, found in collections and on the market, are amblygonites from Brazil. However, stones from Mogi d?s Cruzes, Sao Paulo, Brazil, are montebrasite.

Extremely rare material from Karibib, Namibia shows a lilac color. These stones also make beautiful faceted gems.

Natural bluish green, like that of this oval-cut specimen, is a rare color for amblygonites. 7.47 cts, 14.2 x 10.9 x 8.1 mm, Brazil. ? ARK Rare Gems. Used with permission.

Identifying Characteristics

Species	Locality	α	β	γ	Birefringence	SG
Amblygonite	Chursdorf, Germany	1.578	–	1.598	0.020	3.101
Amblygonite	Uto, Sweden	1.591	1.605	1.612	0.021	3.065
Montebrasite	Karibib, Namibia	1.594	1.608	1.616	0.022	3.085
Montebrasite	Kimito, Finland	1.611	1.619	1.633	0.022	3.00
Natromontebrasite*	Fremont County, Colorado	1.594	1.603	1.615	0.021	3.04

Note that refractive indices and optic angles decrease as sodium (Na) and F content increase. The change in optic sign (where 2V = 90°) occurs at around 60% (OH). A complete series of (OH, F) substitutions appears to exist.

* As of 2006, the International Mineralogical Association (IMA) no longer considers natromontebrasite a mineral species. Now, it’s considered a mixture of amblygonite, lacroixite, and wardite. (See our article on gemological formulas for more information).

Amblygonite: Brazil (5.2, 6.3). Photo ? Joel E. Arem, PhD, FGA. Used with permission.

Synthetics

Using the hydrothermal method, scientists have synthesized amblygonites and montebrasites, as well as intermediate members of their series (Canadian Mineralogist, Vol 14, 357 pdf). However, jewelry use for such gems is unknown.

Enhancements

Pale yellow amblygonites may turn pale green with radiation treatment.

Sources

Custer County, South Dakota produces non gem-quality amblygonite masses up to 200 tons in size. Tinton, South Dakota also yields non-gem material in masses.

Most gem-quality amblygonites come from Brazil, where they occur in masses and crystals of fine, yellow color.

Other notable sources include:

United States: Arizona; Pala, California; New Mexico;
Germany; Sakangyi, Myanmar; Varutrask, Sweden.

Notable gem-quality montebrasite sources include Montebras, France and Karibib Namibia, as well as Brazil.

Other notable sources include:

United States: Newry, Maine (in crystals greater than 3 x 4 inches in size, found in 1940-41, heavily included and provided only small gems); New Hampshire.

When Rob Lavinsky first saw this crystal with intense yellow citrine-like color, he “thought it was a particularly intense orthoclase from Madagascar.” He recalls: “I next guessed a bizarre twinned dolomite from Brumado. I only drifted to guessing montebrasite/amblygonite on try #3.” Montebrasite (Twinned),?Telirio claim, Linopolis, Divino das Laranjeiras, Doce valley, Minas Gerais, Brazil. 6.0 x 5.0 x 4.5 cm. ? Rob Lavinsky, www.iRocks.com. Used with permission.

Stone Sizes

The largest cut amblygonite, cut from Brazilian material, weighs approximately 70 cts. Faceted gems normally range from 1 to 15 cts. However, cut gems very rarely top 10 cts.

Smithsonian Institution (Washington, DC): 62.5 (yellow, Brazil), 19.7 (yellow, Myanmar).
Royal Ontario Museum (Toronto, Ontario, Canada): 15.6.
American Museum of Natural History, New York: 3 (colorless).
Devonian Group (Calgary, Alberta, Canada): 47 (yellow, Brazil)

Care

Due to amblygonites’ and montebrasites’ relatively low hardness and perfect cleavage, avoid using mechanical cleaning methods like steam and ultrasonic. Instead, use a soft brush, mild detergent, and warm water. Consult our gemstone jewelry cleaning guide for more recommendations.

You’ll more likely encounter these stones in mineral?collections than jewelry collections. However, if you want jewelry made from these gems, use protective settings and avoid ring use. Pendants, brooches, and earrings shouldn’t pose too many risks. Also, store this jewelry separately from more common, harder gem materials, like quartz, to avoid contact scratching.