Amphibolite, particularly the variety known as Golden Amphibolite, represents a fascinating intersection of geological processes, metamorphic transformation, and aesthetic beauty. While not traditionally recognized as a mainstream birthstone, its unique golden iridescence and connection to ancient geological events make it a noteworthy gemstone for enthusiasts and collectors. This article will delve into the origins, properties, and significance of amphibole and, specifically, Golden Amphibolite, drawing solely from available geological and mineralogical data.
History and Lore
The story of amphibolite is deeply intertwined with the history of the Earth itself. The dark bands within the Morton Gneiss of central Minnesota, a rock largely composed of amphibole and biotite, contain inclusions dating back 3.6 billion years. While this age has been surpassed by discoveries in the Canadian Arctic, the Morton Gneiss remains a significant physical link to the planet’s earliest stages. This rock has also been prized for its decorative qualities, used as a building stone both regionally and internationally.
The more recent recognition of Golden Amphibolite as a distinct variety stems from its striking visual appearance. Found in locations like Western Australia’s Pilbara region, as well as the USA (Wyoming, New Hampshire, Massachusetts), it is traded as “Gold Leaf Stone” due to its shimmering, golden luster. The development of this color is linked to metamorphic processes and subsequent alteration, rather than inherent properties of the original amphibole minerals.
Geological Formation and Sources
Amphibolite forms under specific metamorphic conditions, typically at temperatures between 500° and 700° Celsius and pressures of 5 to 10 kilobars – equivalent to depths of 15 to 30 kilometers. This occurs during orogenesis, or mountain-building processes, through the regional metamorphism of magmatic gabbro or volcanic basalt. The resulting rock, amphibolite, is characterized by a coarse-grained structure with alternating bands of plagioclase feldspar (typically white to light grey) and amphibole minerals (usually black, but potentially brown or dark grey).
Golden Amphibolite’s distinctive color arises from a partial decomposition of the amphibole minerals. This process creates thin platelets and flakes of iron oxides, specifically goethite, which are responsible for the iridescent golden luster. Similar decomposition processes are believed to be responsible for the color in Golden Amphibolites found in various locations worldwide.
Currently, commercially available specimens originate from the USA (Wyoming, New Hampshire, Massachusetts) and Western Australia (Abydos Station, Pilbara region). The Pilbara region of Western Australia is particularly noted for its occurrences of this unique stone.
Gemological Properties
Amphibole minerals, in general, are characterized by their dark color, hardness, and well-developed cleavage. These properties often distinguish them from other common rock-forming minerals, with the exception of the pyroxene group. The angle of cleavage planes (56° and 124°) can theoretically differentiate amphiboles from pyroxenes (87° and 93°), though this can be difficult in practice, especially when amphiboles have altered from pyroxenes.
While amphiboles can exhibit a vitreous luster, some may be mistaken for metallic minerals. The presence and orientation of cleavage planes are the most reliable method for identification. Amphibole crystals can sometimes display six-sided cross-sections, but identifying them often relies on the overall mineral composition of the surrounding rock.
Golden Amphibolite’s unique property is its golden-yellow surface luster, a result of the iron oxide alteration. The underlying amphibole minerals are most likely grunerite, though microscopic, radiographic, and chemical examinations are required for definitive identification. The mineral composition is not readily visible to the naked eye.
The following table summarizes key characteristics:
| Property | Description |
|---|---|
| Color | Dark green to black (Amphibole); Golden Yellow (Golden Amphibolite) |
| Luster | Vitreous to metallic |
| Cleavage | Well-developed |
| Hardness | Not specified in sources |
| Formation | Metamorphic (regional metamorphism) |
| Composition | Primarily amphibole minerals, plagioclase feldspar, iron oxides (Goethite) |
| Typical Sources | USA (WY, NH, MA), Western Australia (Pilbara) |
Symbolism and Metaphysical Beliefs
The provided sources do not contain information regarding specific symbolism or metaphysical beliefs associated with amphibole or Golden Amphibolite. As a relatively uncommon and recently recognized variety, it does not have a long-standing history of cultural or spiritual significance.
Care and Cleaning
Due to the lack of specific hardness data, general care guidelines for moderately hard minerals should be followed. Amphibolite, being composed of various minerals, may be susceptible to scratching or damage from harsh chemicals. Cleaning should be performed with a soft cloth and mild soapy water. Avoid ultrasonic cleaners or harsh cleaning agents. The altered iron oxides responsible for the golden luster may be sensitive to prolonged exposure to water or chemicals.
Conclusion
Golden Amphibolite represents a captivating example of geological transformation and the beauty that can arise from the Earth’s dynamic processes. Formed under intense pressure and heat, and subsequently altered to display a unique golden iridescence, it offers a tangible connection to the planet’s ancient history. While not a traditional birthstone, its aesthetic appeal and geological significance make it a compelling gemstone for collectors and those interested in the wonders of the natural world. Its formation within metamorphic rocks and association with orogenic events highlight its deep connection to the Earth’s fundamental processes.